Because the 360 version is better due to not needing the dongle and you add in the fact that the 360 has more people, and add in the fact that you can use play Guitar Hero 2, and I think those are all selling points and why Multiconsole onwers will buy it too.
However, the fact that I can use real cash on the PS3 to buy songs instead of MS points is also enticing, but not enough to overcome the other factors. Mainly to play Guitar Hero II, and no dongles.