People wanted to revert her because she was no longer Mercy. Some used arguments as underpowered or overpowered to find traction for their desire to go back to the Mercy they enjoyed playing… I think most Mercy players have acknowledged that she was really overpowered by now. And it wasn’t Mercy players who wanted a rework, and I bet the people who wanted a rework now regret ever wishing for it.
There’s not much to explain really… When Mercy was overpowered it made people develop a hate for Mercy and the people playing Mercy. Gameplay was ruined, and toxicity grew, and now we’re frustrated and angry at the community and at Blizzard for all of it. Current Mercy is by many (not all) perceived to be unengaging and unfun, but since she’s such an important member of the current meta someone in each team is selected for the Mercy chore… because she is a chore… I certainly do not find her fun anymore and I bet there are plenty of people who agree with me. I don’t want a revert though, but another rework would definitely be preferable over an 8th consecutive nerf…