Thought it would be pretty clear why Reach (and other Halo games) aren't on the list.
Halo 5 just came out on the X1. Why in the world would they put a older Halo game, in BC form) on the system so close to their most recent launch? Business wise, it wouldn't make sense. It's bad enough that MCC is on the system to compete with H5. They don't want another Halo re-release so close to a major release.

Now, I'm sure there are other issues at play as well. Like ensuring the emulated version works well with the 360 servers and cross platform play. And what about the fileshare? Will that carry over as well? Most likely, it's mostly because of the conflict with Halo5's release. And with Halo 5's Forge coming out in December, it helps push sales, and players, toward that game instead of a older game.