Well, I know why…
Well, I know why “cunt” is considered a dirtier word than “vagina” or “vulva” (as you mention, it’s because it’s was a vulgar word, in the literal sense), and I’m familiar with feminist efforts to “reclaim” the word. My concern isn’t with profanity; it’s with “humor” that reduces a woman — even one who is, in fact, a really rotten person — to her genitals in order to express your anger or contempt for her.
English affords a lot of ways of insulting people that don’t have anything in particular to do with sex or gender; there’s no need to do it this way.