Often people in the majority position feel like any push to add more diversity is an attack on them. There've actually been studies done where when teachers switched from calling on whoever to deliberately calling on boys and girls in exactly equal amounts, the boys in the classroom felt like they were actually getting called on less than the girls. It's kind of just natural to feel like this stuff is zero-sum.
Then you mix in the political aspects, i.e. "OMG the PC anti-creator race-card-players have forced another white character to be changed to black!"
You can see a similar process happening anytime we have a thread about having more female characters in vidja games. It's not that they're bad people, it's just that they feel under attack and like they have to defend themselves.