To look at it seriously, I think people are mostly able to process that being a woman or a POC isn't a fundamentally undesirable state, so they get why people fight racism and sexism. Ablism, though... well, being disabled SUCKS, doesn't it? So why go to all this effort to promote the bullshit notion that you shouldn't say so? So a lot of people who could mostly put up with anti-racist/sexist activism hit disability rights, which is not something that gets a lot of press or attention in textbooks or anything, and go, "NOBODY EVER TOOOOOLD ME THAT WAS A PROBLEM!" It's kind of the straw that broke the camel's back for them.