I see, weird that they didn't do it already. I'd think that this isn't too hard to emulate in comparison to other xbox 1 games.
Three possibilities:
1. All the 2D games, since they're based on PS2 code, use some kind of funky engine/coding that makes them surprisingly hard to emulate. I think this is fairly unlikely though since other games based on PS2 code (Super Monkey ball, Sonic Mega Collection, Megaman Anniversary, Fatal Frame 1 and 2, Silent Hill 4, etc.) are plentiful on the BC list.
2. It's a conspiracy, just like 9/11. Capcom doesn't want to risk a (still unannounced, and with the speed of games hitting XBLA, means it's not gonna happen until 2008 even if it was announced today) future XBLA port of SF3 or Capcom vs SNK 2. Unlikely because 1. Neither game would fit the current size limit on XBLA 2. Other games announced for XBLA (or out already) do have BC enabled Xbox1 versions (UMK3, Pac Man, Galaga, Sonic the Hedgehog)
3. These games had low sale numbers (which they did) and MS assumes that these games are low priority. So they're just at the bottom of the pile. I don't think MS takes notice of forum posts, which is why the emails are important. The more email requests they get the greater the chance these games will get a higher priority.
So send an email, and if you already have sent an email, get your friends to send emails too.
