I think it was OJ Simpson who said that, when he played in the NFL, he always wore the smallest amount of padding possible, since it inhibited his ability to sense his pursuers as they bore down on him. I always figured that one of the things that Force Sensitivity did was enhance this sense, and that with an Armor shell, a Jedi's ability to be acutely aware of his surroundings would be compromised. So, they'd basically be trading their ability to anticipate and dodge/deflect a blaster bolt for a suit of armor that probably wouldn't save them from that same blaster bold either. And, as mentioned before, no armor's going to save you from a lightsaber.
I dunno if this is the real answer, but I think we'd have seen some more Jedi in armor by now if there wasn't some reason for them not to use it, and that seems like the easiest explanation to me.