Because...
...twat is just a vulgarism like dick or prick. Cunt is a lot more, worse than calling someone a fucking bitch. Let me put it this way, if someone called me a twat, I'd probably respond in kind, calling them prick. If someone called me a cunt, I'd want to kick them in the balls. Palin would just shoot them from a helicopter.
BTW, if you'd seen Carol Burnett on Jay Leno last night, you'd know twat is the past tense of tweet.