Okay. When you're in Boston (where Harvard Law is) people talk with a major accent. Like 'pahk the cah,' meaning 'park the car.' In Legally Blonde, not one person speaks with that Bostonian accent!!
Not all people who go to Harvard Law are from Boston. People come from across the country and the world to go to Harvard Law. Also, not everyone has an extremely strong Boston accent.
She didn't talk to many people outside of the college, and in the college people came from all over the country. They shouldn't be expected to have an accent.
OK, I think that you are thinking of a Brooklyn accent. I live in New York and I have been to Boston many times. They don't speak like they are from Brooklyn, and neither do the people in my area.
None of the movie was actually filmed in Boston. It was all filmed at USC or various places in southern California. They talk about it on the commentary. Plus, you don't have to be from Boston to go to college there.