The armor limitation in the game was dictated by encumberance. Given a poor implementation of the encumberance stat, they opted to remove that stat and that left a hole for armor. Armor should be a limited to particular stronger "classes" and particular "weaker" classes should be limited. Thus, was born the Armor proficiency and the chain armor proficiency, and the plate armor proficiency.

I will not debate the merits of either system, just pointing out the overall change and to why it happened.