Thats not the problem. Dark Knights have used axes in the past. The problem is they've never been restricted to ONRY AXUSEU!
It really is going to be sad when some jobs that are supposed to technically be masters of more then one weapon can't even do that thing thats part of their Final Fantasy History and lore. (Looking at Fighter/Warrior, Thief, Monk, Dark Knight, Dragoon, Ninja, Knight, Paladin, etc.)
Also Knight is not always "Paladin" Knight in FFT was simply the user who had access to special weapon and armor destroying abilities. They were a more advanced form of Squire. You could turn them into Paladins by equipping white magic as a sub skill.