"It's a lot of different things," explains Jason Sussman who is the senior environment artist at Bungie. "I could get really technical, but I’d bore you to death. A lot has to do with performance. We’re cross-platform, so we had to be very delicate with all four consoles, making sure they have the same experience, that it looks as good as it possibly can on all these consoles. A lot goes into that, from geometry to textures to post effects to particles to how many players you get on the screen. We’re always balancing that out to make sure everyone gets the same experience."
Parity. Terrible, terrible parity. The sooner these cross-generation games are done being developed, the better off we'll all be.
I assume the primary version of the game is Xbone or PS4, and that there are other teams doing the majority of the port work, so what would hold the main team back?
Parity, that's what it is!
Or a developer who has yet to release a game running at 60fps, the PC Halo ports are by other teams.