It isn't a matter of HMX not wanting to port every song over - it's obviously a matter of economics.
Let's say HMX did plan to move over every song from the RBN to the other consoles. They could automate the process, but it would have a cost to it- time in porting/download fees/accounting. They could easily build that in - each band/author when submitting would check a box that says something like "Make this song available on the PSN and Wii at a 15% profit, instead of 30% on Live". This is of course a beneficial move for the band - more sales, more publicity, zero additional cost. You could expect that every band would always check that box.
However, for songs that don't sell well, this would lead to HMX losing money. The additional money off the top they charge might not cover their fixed cost of handling the porting for each song.
The XNA has made the HMX centered costs for producing and releasing RBN tracks low enough to make it (hopefully) commercially viable. Currently, the PS3 and Wii don't have a system in place that allows enough of the costs involved to be handled by the artists/authors/volunteer testers - so putting songs on those systems that might only sell a few hundred units doesn't make sense for them.
For those of you claiming that HMX should have just developed a system that would work like XNA for the PS3/Wii, realize that if they had infinite money and no need to make a profit, they probably would have- but it's out of the scope of what they do (Microsoft put a ton of money into the XNA, which gets paid back to them by all those Gold Live members).
I'm a PS3 owner and my excitement about the potential of RBN was only equaled by my frustration that it wasn't happening for me- but the way it's working out makes sense. I'm hoping that we see a great majority of the RBN songs eventually get brought to the other consoles, but it sucks for everyone involved (players, bands,authors,HMX) if they don't.