Because in that context, the word "bitch" is used as a derisive synonym for a female who has apparently done two things, 1) lost a contest and two) is female.

It was not used in an ironic way or general way. It literally was used in place of the word, "She". The word bitch, while being appropriated for uses beyond that of a slur against woman, or reclaimed by women themselves, cannot always be casually used as a synonym for female pronouns by males without there being an undercurrent of misogyny. And what's worse, is that many people will not even "get" it.

And I brought this up because MILF seemed gleeful at the idea of me or foa/rubah/shlup exploding in rage at something overt and ridiculous, when it's far more frustrating when men display casual sexism without even realizing it. At least from my perspective.