There are four main reasons why the Minang have remained untouched. The Minangkabau are culturally distinct. They do business and interact casually with outsiders but in matters of personal and social identity, even among the over two million or so who are outside their Sumatran homeland, they are distinct and want to stay that way. Minangkabau are taught about Christianity. They are taught, for example, that Christians believe in three gods, god the Father, god the Son, and god the Mother.
Western media floods their homeland and they smugly love it.
For example, the average Minang man on the street believes Madonna is a Christian. And in the way that they regard themselves as Muslims, she is. Muslims are born that way. If they convert they change their name. Madonna has a Christian name, therefore she is a Christian.