It seems counter-intuitive, but being able to trade the items would adversely affect the system. For one, as Ghostcrawler mentions, one of the points is to alleviate any pressure from others in the raid to give your loot to someone (supposedly) more deserving. If you have no choice - no pressure.

Secondly, with this new system, everyone's roll is independent of everyone else's, so if I understand it correctly, in a 10 man raid, 10 people could, theoretically, win loot off of one boss. Some of that loot will not be usable (because it's a duplicate or not an upgrade, etc.). If trading was enabled, it would screw with the odds of loot distribution because something not useable to you might be useable to someone else, even though they technically did not win it, so they would have to lower the likelihood of loot dropping to keep it from giving out useable loot too quickly.

It's just like with any other soul-bound item that you get from quests or rep. Sure it would be nice, sometimes, to be able to give that to someone else, but that would screw with the distribution tables of the game, which is why you can't.