This.
M+ was playing the equivalent of Candyland, and being rewarded with the best loot in the game.
This is a move to make the rewards more in line with the challenge. Which is the central focus of RPG design, and has been since they were created in the early 1970’s.
(It won’t last, though. Blizzard will cave and nerf M+ difficulty into the ground, while leaving the inflated rewards intact.)