1) The console is the hardest to develop for. This doesn't mean the software has to suck, but it does mean developers need to put in more effort than the minimum amount possible.
2) It isn't the lead console, so developers may not feel that they should invest as much money into making the game up to standards (see point 1, they need to do more work than absolute minimum)
3) developers are lazy and/or running out of money. With all the losses being posted, they may be under strict orders not to go over budget. This would prevent them from fixing problems that go above and beyond point 1.
@question tacked on end
It is best to design first on the most limited system and then port to the more capable system. This however is very hard to figure out in this situation. You have less cores (therfore less threading) on the X360, so from that standpoint, it makes sense to develope on the system with less threads, and less complexity and then port up. However, the X360 is more advanced in other areas (flexible memory) so it would make sense to start on the PS3. Obviously I don't know enough about both peices of hardware to make an educated decision, but the general rule of thumb is start on the limited hardware and port up. Also, there may be other factors like dev software quality that factors into which system to start on.
