Because you use both arms to pick up the key so you don't have an arm free to pick up your shield. No picking up shield, no force-field. No force-field, no shielding.

And for the over-obvious game-play answer, picking up a key places you in a higher state of vulnerability. It slows you down aswell if I am correct. Running around with a key is running around with something highly valuable for that level, so why should you be able to still act normal with something like that?

first, kymroi, i don't think he suggested shield while piccking up a key. He said exactly that the key let you vulnerable (and could draw a target on your head, as a manner of speaking....translating: in a group, who is carrying the key will be the main target of the monsters. At least the more inteligent ones).

Them, to explain their fire protecting properties: if you consider that our knights don't breath, then, the shield also function as a cup around a candle, extnguishing the fire when the oxigen is over.
About spikes: yes, you can walk over then while shielding, but this is useless anyway. You will most probably get your shield broken if you try to pass a floor of spikes with it.

You can't attack when you're holding a key anyway, so why should you shield when you're holding the key?

This only encourages more party play. The person holding the key will be protected by their partners (for the most part) and their partners don't have to worry about attacking monsters before backtracking to get the key.
And if you're a hardcore gamer, the time it takes to put down a key can cost you damage that you could have prevented if you went with a group.

Spacefaring knights or not, I'd always imagined that the shield technology had been stolen from the gremlins, and it was this act that rendered them so hostile to the knights.

If you'd like a story-based explanation for the inability to shield a key, I propose the following: the shield strength is inversely proportional to the area it covers. (hence, the half-shields of gremlins are totally invulnerable, whereas the 360 degree shields of knights can eventually be broken) If we posit that the shield must extend to cover the entire knight (and anything they are carrying), then for the ginormous keys favored by the clockworks, the shield would need to expand to nearly double the normal volume, thus rendering it too weak to be of use.

If you'd like to make this relevant to gameplay: I'd be interested in seeing shields with different behavior, like more strength in exchange for reduced coverage. (though since the technology is stolen, you'd still never equal the total invincibility of gremlin shields) In particular, this would be useful at near-core levels, where even a single blow from a fired devilite is enough to shred many popular shield types.

@pauling

Well, you get the proto shield in the start and that exists prior to the crash so that would go against you tech stealing theory. Though those strangers do sell shields I would expect those things are fairly universal.

@Shango

Something like a emergency defensive system that acts like bomb?