Religion played a key role in this too, as these areas were the center of the Covenanter movement, whereas the north of Scotland was generally seen as anti-Covenanter and pro-Catholic.
Don't think that pro-Catholic is correct, though there are certainly pockets in the northern part of Scotland where Catholicism survived despite centuries of oppression.
According to Wikipedia Catholicism was outlawed in Scotland from the time of the Reformation in 1560 until the late 18th century.
https://en.wikipedia.org/wiki/Catholic_Church_in_Scotland
