Fantasy rednecks

Because both are proud and mountanious nations?

Apparently, Tolkien's dwarven language just naturally works well with a Scottish accent. Earlier radio, tv, readings leant into this and other writings including fantasy dwarves followed suit.

It just sounds right.

Dwarves have American accents in Dragon Age!

You can't pick a Scottish accent for your character in Baldurs Gate 3 and now my Dwarf just sounds British. It sounds, well... off