There is just no one playing competitive at this point.
First and foremost, Reach is 10 years old and was almost universally disliked by competitive players from day 1, so our cumulative patience with it probably started low.
This is compounded by the fact that after level 20 it becomes basically impossible to win a DMR 1v1 against controller players thanks to the massive aim assist. Very easy to just get frustrated and stop playing. My self and the dozen other competitive Halo players who moved off controller long ago had this experience. I assume we aren't the only ones.
I also assume that since the playlist became 90% controller players at that point, they likely just went back to console MCC which has better games than Reach and a higher player count in Reach.
Big bummer. Hopefully the next release has a better solution on release :(