I was thinking about this, and the reason is obvious.
360: Played on xBox Live
PS3: Played on Sonys network (forgot the name)
Wii: Two local players no online support
PC: Requires a server.
The idiots behind the helms here simply didn't want to bother setting up a master server. Maybe if enough code is left in from the 360 it will be patchable for a private network.