Speaking to my M+ friends, part of it could be also the selection of dungeons (seems they are not that popular).
And while the change they made to the difficulty make sense for people who enjoy doing higher keys, for the more casual (as in, not too serious) players who may have enjoyed the lower keys, or those wanting to get into it, the learning curve may have become too steep.
I know that during season 1 & 2 of DF, a friend and I enjoyed doing M+ 2 through about 9, gradually going up as we geared up and felt more comfortable. (Season 3, an old friend came back and kindly dragged me through a bunch of M+ to get a lot of good gear - he was good enough player to carry my sorry @$$ and I was able to complete I THINK even a few 20s). But after they made the change in S4, I did very little M+. In TWW, I don’t think I’ve done one.
THOUGH I’m sure delves had had a big impact, and lower subs certainly will have affected many group content.