I'm not sure why Hollywood casts against type so much. If they need a British actress, hire a British actress. There are plenty. Don't get an American woman to do a British accent, it sounds horrible.
It's so weird. British actors like Bob Hoskins and Hugh Laurie can do a perfect American accent, but Americans are lousy at British accents.

Probably to create a considerable distance when Sylvia planned to take Mary away to her family. If she lived in the states, the guys might not have been as torn up about the baby leaving.
That said, I also agree that her accent was horrible.
Goonies (and neomaxizoomdweebies) never say die!

I agree.
Nancy's "English" accent was atrocious
"The Opener of the Way is Waiting"

Probably no significant reason. If anything, she should've been French as this movie is a remake of a French film. I guess they figured if Mary were traveling far away, it would be a much bigger separation for her and the men.
The real question should be why didn't they hire an English woman to play her instead of an American woman with a bad British accent? Was Emma Thompson too busy or something?