1. HMX has to recode every song moved to make it compatible with the PS3. That's a lot of songs and a lot of hours...which costs money.
2. Speaking of money, I'm sure the QA and approval process to get the songs on the PSN Store isn't free. More money.
3. Though the specifics of which song will make it over hasn't been released, it's safe to say that the songs not doing well on the 360 will likely not come across. The 360 fan base is larger than the PS3's (I'm guessing really, but I'm pretty confident) but I'm sure the what sells well as far as regular DLC goes is pretty similar on both sides comparatively, so it isn't likely that HMX would risk spending the time and money to transfer something over that might not give a positive return. I'll admit, this one is pure speculation on my part.
4. Ok, so there's really only one reason...explained 3 ways.