That’s in part due to the continued development of the game, which has spread to the PC platform as Microsoft sticks to their plan to expand their ecosystem beyond the Xbox family. Given how convenient the whole launcher system is, it makes it easy to sift around and dive into whatever sort of experience you want at any given time. It’s still a marquee title for the Xbox brand in that it basically collects every Halo game ever made in one place: and it’s still lively to the point where I can jump in near-weekly and get into a game quickly.
You really got to give it to 343 Industries and their support of the Halo: Master Chief Collection.