HMX needed a system that allowed players such as us to create and chart our own songs for the game, without HMX's help.
Microsoft was the only company out of the three that had this system up and running (the XNA creators club).
So HMX used this system to create the RBN.
If Sony or Nintendo had something set up instead of Microsoft they would have used their services.
But as it is, only Microsoft had the right program already created and running for HMX to use.
That is the reason it only works on 360.