They also weren't yet in the mindset they're currently in where every Xbox exclusive is also play anywhere on PC. Same reason Sunset Overdrive isn't on PC.Honestly, that's a fair point. Not like to go all conspiracy on you, but it's very possible a few years ago they wanted to put MCC on PC but knew it would never fly with how poor of a product it was (technically). So they probably waited till Xbox One X for a 4K update and for Halo 6 to be on the horizon. Now is probably a great time from that perspective. So kind of a, hey look how many birds we can hit by a fixed MCC.
Just spit balling. Wait till some "news" site turns that into a "rumor" tho lol.