Okay, here's the reason...
Xbox has a system setup to accommodate uploading of game files. PS3 does not.
Harmonix can't develop for something that doesn't exist. There is nothing HMX can do until Sony give us a way of uploading content (and this is ME saying this, sa_nick the PS3 fanboy)
I assume it's the same for Wii. It has nothing to do with who deserves it, everyone deserves it! it's just that it's impossible to give the feature to non xboxers.