The most obvious reason PS3 won't get all the songs is money.
I would imaging that some sort of QA has to be done for each track, which someone has to pay for, prob HMX. I doubt Sony would allow stuff without testing on its system. This is already handled for the XBox by the peer review process.
There has to be some payroll system set up. Right now, MS takes care of this for sales on 360 (actually I believe they farm it out to a third party). Sony does not have this so a company/personel will have to be hired to handle this for PS3 sales. This will also probably be done by HMX.
So in order to cover all the costs that do not occur on the XBox, HMX will have to be sure the song will sell enough. Releasing a song that will not recoup the costs of Testing and/or Payroll will not be a viable business decision.
* This is all speculation on my point, but it makes sense to me.