Well as im not part of the game making industry in any part I may be a little off but I would say its because these games are programmed to function on the various consoles first and foremost as with titles like Halo being a flagship title.... I believe the games have to then be reprogramed so that they function on PC taking into account of the multitude of hardware configs out there and adjust the games internal workings accordingly would obviously take time..... and as we know games like Halo 2 etc are just an after thought for the PC