The UU and NU communities have been thinned over the past few years by the lack of objectives on ranked and the constant nonsense meta changes.
I'd say the first one and a half years of matchmaking after the release were decent in terms of finding matches, so like season 1 to 6.
As Incognition mentionned, the only time you'll find people playing a lot is in the first few weeks of a season and the 2 weeks before seasonals pretty much.
Never expect to play ranked when PVE events are going on, I had some year where it took me 8 days to find a single game (basically the whole event) to be able to not decay