There'll be no accurate answer for your question, but guesses. My guess is:
1 - Not enough interesting from Linux community to port those games. Most Linux users aren't gamers...
2 - Technical difficulties: Even if the game uses Unreal engine, it could still be rendering in DirectX instead of OpenGL and since those are closed source games that will be no easy task. Also, some games, like Unreal 2 is so buggy that you should be happy to get it to run sometimes.
3 - Some games are just so crappy that the trouble porting it is not even worth. XIII has to be the worst game I ever played. Full install requires you to swap disks during gameplay, for example