Why is Spiral Knights Based on offence?

It leaves out most of the rest of the content in the game.
And I'd kinda like to know why.

I wouldn't mind a more defensive focus... but my question is how do you do that?
Buff armor sets that don't increase offensive abilities? Alright sure that could work, but it will still always be better to just not get hit, instead of withstanding a hit. I see people running FSC in full Skolver, because they just don't get hit enough and gain too much from the +damage.
Buff certain shields? Well then what prevents Wolvers from equipping said shields for the defensive boost as well as their own offensive boost? Also, people have already forsaken Shields in general for Swiftstrike Bucklers (offense boost).
Nerf offensive armor sets? Beyond the obvious backlash OOO will get from the playbase, it'll just kick dirt in the faces of all the people who like playing offensively. For those who like playing defensively, they'll still be stuck with armor that can only take a few swings in tier 3 before they're lying flat on their back.
You can debate heavy swords being better defensive weapons than offensive weapons though.
The only way I can think of maybe adding a defensive focus is maybe adding like a parrying mechanic, that relies on timing to negate or even reflect damage, encouraging players to wait for an attack to hit them, instead of launching an attack first.

I always thought SK was more of a "ninja" based game. Avoiding hits and attacking when the opportunity grants it
Anywho, as far offence is concerned, the faster something is killed, the faster you can grind and make money. Defence based armour is also useless if you don't get hit. Most players prefer killing things faster and making up their lack of defense by dodging (besides, something can't hit you if they're dead, right?)

Maybe Defensive sets could give a slight invulnerability to damage? Instead of dodging perhaps I could rush in on vana in full plate... take a bash to the skull, a slag in the chest and still give him some thrashing?
Mixing and matching armors could become more utilitarian than just costume?
I mean... really... Speed isn't everything... sometimes we enjoy just being a tank as much as killing everything dead too.
Unless... you know... you guys just enjoy laying on the ground every time your lag jumps on you...

defensive play style but support the other party members, me very really want!

Problem: Source of damage (a monster)
Options: Run away, tank it out, or kill it.
Best/ Fastest option: Kill it.
That's why the game is more offence oriented.
The majority of the monsters die somewhat quickly. As said above, best way to survive is ruuun. Getting hit, even with great defenses, is still gonna hurt, especially in t3. I guess SK defense is based more on crowd control then actual getting-hit-and-tanking it.

Lets take skold's example...
If plate let you take Crap loads of damage, and boosted your Health, increased aggro per hit, as well as lessened or removed knockback and being interupted...
You could run right up to vana, laugh at the fire bouncing off your armor, while diverting its attention so that the gunners and bombers can pummel it...
You could use heavy weapons to herd and aggro a large group, while Taking all the heat while a Glass cannon hits them from a blind spot.

SK would feel too much like a stat-based (MMO)RPG if we were able to actually tank like that :L
Grind for the best defense, farm even more for the ultimate defense boosts (lol, almost sounds like UV/heart trinket kids) and all that just so you can aggro and tank all the monsters.
I have played that role in other games I played before SK, but idk, that kind of behavior doesn't feel right in SK.
You can play just fine (and defensively) with all the other gear - I used to feel like I was the defensive player using my then-Khorovod while my friends had their Brandish lines. I got so used to having knockback that even now, I cannot use 3-hit swords for the life of me.
Anyway, I'm really unable to relate with this.
Now, if you meant to say Why is the Spiral Knights community based on offense, I can understand, haha. Optimizers are in any game.

Tank and spank is so easy though... I like SK because it doesn't have those traditional RPG class roles.
Also, if you can withstand Vanaduke hits, think about normal SK. You can just sit in place while an entire arena worth of monsters surrounds you, flattening them with a few heavy sword swings. No skill whatsoever, just spam click (Auto-target and wide swings can compensate for the rest).

"You could run right up to vana, laugh at the fire bouncing off your armor, while diverting its attention so that the gunners and bombers can pummel it..."

"Buff armor sets that don't increase offensive abilities? Alright sure that could work, but it will still always be better to just not get hit, instead of withstanding a hit."
Yeah, but. . . that's okay, right? The ninja gods with great pings would carry on as always, but those of us less gifted and/or fortunate would have more options.

Perhaps it's because SK is inspired by old era games. As far as I remember in that sort of games you weren't supposed to tank - you had to avoid damage either by dodge harmful objects or actively block them (or destroy sources of damage). I'm thinking about games like Mario or Ghosts'n'Goblins, where one hit actually made you dead. Such design promote player's reflexes and skills - perhaps that's why defence is rather focused on shields the armour. We have to actively block with shield (and that require reflexes and timing). Shields are not just objects that improve tanking stats passively. Let's look at barrels in Donkey Kong - you either jump over them or destroy them with hammer - there's no way to tank barrels.
I know SK is entire different game then Donkey Kong or Ghosts'n'Goblins, but I suppose the developers wanted to create similar feeling.

Hmm. Merethif has a good point, but that leaves Royal Jelly armor and other such armors without a projected use. However, suppose there were features added to currently "defensive" armors?
Plate lines could lower the effect of knockback and interruption on a player, meaning you can still swing your sword even after the Wolver bites you- it has an ASD low to make up for this affecting ALL incoming damage types' interruption and KB. Ancient Plate would have this in abundance; it also has an MSD low.
Cobalt series armor would do this only with interruption and KB caused by Normal damage, or split Normal damage. Jelly would do this with interruption and KB caused by Piercing damage, or split Piercing damage. Grey Feather would do this with interruption and KB caused by Elemental damage, or split Elemental damage. Dread Skelly... same thing with Shadow. And other armors will emulate this. Almirian Crusader can copy Dread Skelly, Ice Queen can copy Royal Jelly... maybe in higher quantities, to make the Shadow Lair armors a little more appealing. Improving the effect of MSI on Mercurial armors, or even giving the standard Mercurial armor set a Medium bonus per piece, would also be nice. (Unsure if the latter should be applied to Merc Demo, seeing as it's already amazing for a well-geared or Recon-Cloakin' Bomber.)
Damage bonus vs. Type also could use a buff, and this can come in the form of allowing enemy-specific damage bonuses to stack in higher quantities. Buff the Type-specific armors to have High bonuses vs. their targets (except, of course, for Deadshot, because of the Gun ASI low) at their highest level (Dragon Scale would be High, Wyvern Scale would maybe be Med, Drake Scale would remain low). Arcane Salamander, seeing as it's a Shadow Lair armor that needs some love, might get Med vs. both Beasts and Slimes. Then those Bonuses vs. Type would stack up to EIGHT TIMES- Maximum would be replaced with Extreme, followed by Incredible and then Maximum. Combined with the right weapon, and adding a Gremlin-damaging armor (my idea is "Emberlight Garb" and "Emberlight Hood"), this would make them the go-to armor for whatever monster takes your fancy! Also, Divine Mantle could have a Fiend bonus as well as the Veil.
By the way, the bonuses would only stack with REGULAR damage bonuses up to the old Maximum. So if you have full Mad Bomber and an RSS, the damage would be the old Max against Fiends (shown as Extreme) instead of Incredible. If you switched one piece of Mad Bomber to Volcanic Demo for FSC, you'd still get an Ultra bonus against Fiends.
To increase their use in PvP, simply add the full effects of a damage bonus vs. Type to certain armors. Keep in mind this wasn't my idea- but it's really good and I'll keep quoting it. See, each armor can be likened to a monster type, either because it is good against them (nobody has a damage bonus vs. Angel because the Knights aren't evil (for the most part, though I have to wonder about some knights who post here WHO SHALL REMAIN NAMELESS), but for the sake of balance we'll need to classify the Angelic line), made from them, related to or like them or their garb, or simply fitted in to fill a gap:
BEAST: Wolver (duh), Chroma (duh), Drake Scale (duh), Fencing Jacket (it's made of scales), Spiral Scale (it's made of scales, also) and all upgrades.
SLIME: Jelly (obviously), Quicksilver (obviously again), and all upgrades.
CONSTRUCT: Breakers (looks mechanical enough), Plate (it's got a lot of metal in it, lol), Spiral Demo (fills a niche), Cyclops Cap (looks like an awesome robot head) and all upgrades.
GREMLIN: Gunslinger (it works), Masks of Seerus (do you even need to ask?), Emberlight series (my idea for an anti-Gremlin armor, and works with both that reason and another: it's a Gremlin-made armor anyway), Vitasuits (feels like it'd go there) and all upgrades, except Gunslinger's Deadshot series.
UNDEAD: Skelly (duh), Silvermail (duh), Almirian Crusader (seeing as all the Almirians are dead...), Deadshot (deviates from the Gunslinger series, for obvious reasons) and all upgrades.
FIEND: Magic Cloak (Divine Veil is super-effective against Fiends, and the rest of the line can follow suit), Angelic (Fallen armor obviously fits, and the rest will work because they're super-effective against Fiends) and all upgrades.
This will definitely give you a reason to keep your Beast VHigh GF or even DA! Also will breathe life into the CIV, and other enemy specific weapons like it... though I fear it will be followed by an RSS nerf.
Wait! What about Cobalt? Since it otherwise has nothing going for it (except possibly my Normal Interrupt/KB resist idea), and is a "beginner's" armor for the most part, I figure it could have a little niche where it's immune to Damage Bonuses VS. Type. Proto can have the same thing going for it.
Hope that's a good idea! Maybe I should re-post it on the Suggestions forum.
Perhaps if we give them some time... that'll change... you know... after we buy 1 mil more ce or someodd.