I read on here in multiple threads that Sony charges developers for space on the PSN. WIth the Rock Band Network being able to upload 10 songs a day? Seems that could add up quickly. It's things like this and probably a few others that factor it to the actual decision and made it a no brainer to choose XNA.
Also consider this has been in the works for over a year. Sony & Nintendo probably could of put something together in that time but I'm guessing them getting the "select tracks" each month is just fine with both parties.