Well I think the reason that Halo 3 never went to PC was that when Halo 2 arrived, two and a half years late, without any hype or marketing push it was pretty badly panned. Gamespot, for example gave the xbox version 94% and gave the Vista version 70%.
I do think the first Halo was a revolution creating a whole slew of innovations such as first person control that works on a controller, regenerating health, vehicle sections, arena style rooms where you could pick your own ways through, co-op, console death match and on and on. But I don't think the sequel innovated in the same way and compared to what had been released in the interim (Half Life 2 and S.T.A.L.K.E.R in particular). I can't comment on 3 as I haven't played it.
But Halo has become too much of a franchise for Microsoft to spread out even if only on to its own bally platform (Windows).