There was a rational reason given for Wizards not to wear certain armor in an old game called DragonQuest. It posited that iron interfered with the process of casting spells. Hence the Wizards in that game were limited to using armor made of bronze or leather. It's game-world specific of course, and there's no reason why other RPGs need to follow suit. But it does make for an interesting mechanic: if you want to prevent Wizards from escaping justice, you clap them in irons.