Because most of the games suck. NHL 2K6 isn't much better on the 360 than the regular XBOX, NBA Live 06 sucks graphically (animation collisions), even games like "Gun" say "visuals don't take advantage of the xbox 360". Plus, well, there are "issues" with it..:)
I might wait for PS3, even though we might not see it until the end of this year. Here are some details.