One word ... marketing.
Microsoft wanted an exclusive title for the 360 ... so they paid bungie a bucket of $$ to get this end.
Games == consoles ... exclusive games == xbox consoles.
Bungie have now moved to activision - again another bucket of $$ [maybe, check how MW2 makers are going with that].
They have already stated multi-platform is high on the wish list, PS2/PC/XBOX ... maybe.
PC gaming is alive and well. Easier to upgrade, no RROD, no dodgy optical drives, full HD [none of the 720i == HD crap].
Console is fine for FPS, but anything more it struggles.