Why is it so hard to fill party spots in Tier 2?
Like, I'm constantly doing Jelly King runs these days for money and heat, and I hate running through Tier 2 solo. It's not hard, just tedious and annoying. Yet whenever I try to start a party and get moving, nobody ever joins. I leave it open to all, unlocked, and yet nobody ever joins, at the Moor or on an active level. And when I join a random party, it's either only got one person, or someone leaves and the spot never gets refilled with a replacement.
What gives? Why are Tier 2 parties so hard to form?
"I'm constantly doing Jelly King runs"
Answered your own question.
Yeah but the few times I've done other gates, still nobody to fill the party slots.
I don't get it. Are Tier 2 players in shortage or something?
I do the same thing, and no there most certainly isn't a shortage of players to run Tier 2, when your party is formed it is put in a que behind every other currently open party, when players click to join party it places them in the party that is at the front of the que, and when all the other party's have been filled and your party is next in line I assure you your party will go from just you solo'ing, to a full 4 player party within moments.
I've almost never had a problem getting into full parties for a jelly king run almost immediately, unless it's 2:00 AM. Maybe ordinary/random tier 2 stuff, yes....

I generally don't have a problem getting a party started when I start one. Heck, I randomly opened a party slot for a friend and then three other people showed up (d13). Sometimes there's a few troubles getting a full party, but not usually.
Though you might find me starting a party less now since I chain friends whenever possible ^-^
I think people are afraid of the Jelly King.
And now you know why guilds are good. (Active ones, I mean.)