Clerics are members of the "military" arm of a church like Inquisitor, Witch Hunter etc... and no ordinary clergyman, monk or nun. They are unwavering from a roleplay aspect so they have no Oathbreaker mechanic.
A paladin is more worldly, has maybe influence in the church but no position because the have Paladin orders. The Paladin can waver in a roleplay aspect and loose the support of his deity.
From a deity perspective Paladins are maybe "cheaper". Why channel divine Power in Clerics when you can have a warrior with a sword who fight for the same?
