I think it's mainly because of the price of Live Gold - who really wants to pay even more? Also, the lack of keyboard and a mouse are rather limiting. Of course voice communication is often easier but sometimes, I guess, it's not. Oh, and putting the keyboard in a good place could be a slight problem too. And the lack of buttons would seriously hurt the gameplay due to the lack of different skills...
I thought there were only few yet major reasons; it appears there are actually quite a few.







