The reason why riot won't revert the mordekaiser rework
is probably because all the negative feedback and hate isn't actually costing them anything.
People have been angry for sure, but I don't think many people have actually quit league or stopped buying skins just because of the mordekaiser rework. Riot is a company, and I doubt people being angry at them really translates into lower revenue. As long as people are still playing the game, do they really have a reason to care?
It's the same thing with balancing in general. People are often unsatisfied with the balance team, but nobody (or almost nobody) is quitting just because riot is nerfing and buffing the wrong champions by the wrong amounts.
I think it's a sad truth that unless it's something that makes the money stop flowing, riot can do whatever the fuck they want with the game no matter what the players think, which sucks because it means that riot can just ignore everybody's feedback when it suits them.
That's just my two cents though.