I think a lot of the more casual M+ people checked out mentally after 9.1, because it's fundamentally the same as 9.0, except for shuffling around affixes, forcing people to play both Tyrannical and Fortified for KSM, replacing pride with anima powers from Torghast. No new dungeons, and some things like the domination socket system being raid-only. On second thought, it feels like M+ got relegated a bit to second tier content.
Something similar, although to a lesser degree, happened after 9.0: At first covenant choices were more in flux, players choose what they wanted. But then the casuals left the building after leveling and playing through the campaign, after which the covenant meta solidified pretty quickly.
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