Because they think non-fat, pretty, mentally well, hetero (or even like, just plain old gay and bi) women and black women are not oppressed. When, hello, racism and sexism still apply.
They can't just fight racism or sexism. They always have to tack on some queer shit for it to matter. Because they're misogynistic and racist, end of.
See also: why men imitating women isn't seen as offensive. Because women aren't oppressed duh.
Notice how the word sexism or misogyny isn't used (just transmisogyny, a made up thing), and there's your answer.
Also.. extreme jealousy of people who have their shit figured out. They remind them that they too could stop being fat losers at any time, that it's not just someone else's fault.
'Thought leaders' of Clown world etc