RA can be partially or even completely negated by high accuracy weapons, high accuracy veterancy bonuses or high accuracy at close range.
Armor protects against all small arms weapons (except a few that have higher than the standard 1 penetration, like the LMG 42) at all ranges equally.
It has a unique gameplay characteristic and it fits their model.
Ah I see. And SMGs probably aren't going to be doing a lot of damage even in close range against some heavily armored opponent I suppose.