Unfortunately, a large amount of the remaining MS games have quite a lot of potential licensing issues that could be preventing them being BC. Because they're not altering the game in any way, a single car or music track for instance can prevent the entire game from showing up. Remember how Sonic CD on XBLA has no vocals for "You Can Do Anything"? For a game like PGR4 a single case like that with a single song could be why we don't have it.
Crackdown for example, has a ton of licensed music when driving.