it's not just because they're not on Steam, although that is a factor for some people. currently their games are a mess. they sure aren't fulfilling the claims they've been making for years about dx12. it's amazing to me that they could be so ignorant of the wants and needs of PC gamers
they probably should have started with the free Forza game. but they didn't, and now they're in an uphill battle to prove they can do better. and if they can't do that quickly, PC gamers will throw them in the bushes. they already had a bad rep because of GFWL
and Halo also has something to prove, since it's recent iterations haven't exactly been well received. I definitely don't think a Halo game on PC is an automatic success. it has to be a good game, that functions properly, and even then there's a lot of comp on PC