Why is the Boston accent so darned difficult?

It's well-known that good Boston accents in films are few and far between. When you list actors who can do it, you've got Matt Damon, Ben Affleck, Seth McFarland, Dennis Leary. Even Eliza Dukshu is only okay. And they're all FROM Boston. Heck, I can't do a good Boston accent, and I've lived here my entire life.

It's easy to list movies with terrible Boston accents -- but are there any actors who can do good Boston accents who aren't from Boston?