Because OW is a PvP. There should be a common denominator to measure each player’s worth to decide winner and loser.
Variation is good, but it should still be bound to such common denominators. Mercy as a hero has always been a deviant and too far away from the norms compared to the rest, so much so that she just simply plays her own game. Mercy players can do great thing, but it s just her thing, everyone else aint even trying to compete against her directly.
This is just my own experience and personal bias, but how often do you see people take a Mercy at similar rank seriously? Or do they think of her as an accessory? I like playing Mercy from time ti time as well, but for the integrity of the game, she should have been the nichest among the niches.