The live service had a pretty bad year last year. The linked article goes into great detail but I think it can be summarized in a few points:

• The game shipped without campaign co-op or Forge mode. Forge in particular was a big part of the longevity of prior versions of Halo, and having that missing did them no favours.
• There was a battle pass, but it had a rough landing. Players couldn't select the map they wanted, so for some battle pass objectives you had to rely on random matchmaking to achieve your battle pass goals.
• There was a pretty serious content drought last year. Relatively few new maps were added by Infinite, compared to content release timelines for prior Halo titles, and users couldn't build their own maps because Forge was still missing. 343 originally estimated 6 months to finish Forge, but it actually took a year. They had to keep extending the "season" for the aforementioned battle pass, likely because they didn't have the next round of content ready when they expected.

I think the content drought was the biggest problem. With only a relative trickle of new content from 343 and absolutely zero community content (because that would have required Forge), there wasn't enough to retain player interest.