1- She's a convenient and practical antagonist.
2- Most writers are geeks. They got beaten up by the boyfriends of cheerleaders while said cheerleaders watched.
3- As for why the bullies aren't the antagonists, it's more fun to write about a sexy woman, especially seeing as there's a lot of people here into TG and other transformations.
Mind, I'm a Brazilian geek. No cheerleaders in our schools.