Why is the word ‘cunt’ so offensive in America?
Because in America, as distinct from the Commonwealth, cunt is a reductive description of women, when used as an epithet. In the Commonwealth, the epithet mainly refers to men. It is certainly strong, but it can and is used jocularly, and even as a coarse affectionate term (if qualified with an adjective: e.g. clever cunt).
So in the Commonwealth, the word violates one taboo, and a minor one at that nowadays: sex. In the US, it also violates the much more salient contemporary taboo of misogyny.