Again (apologize if I come across as flippant), 3/3.5e also had damage reduction as a mechanic that worked together with AC. I actually don't mind armor helping to avoid the attack rather than reduce its damage by default - the implication is that something like the full plate just makes most blows glance off, and whatever bypasses your AC finds a weaker spot in the armor / gets past your shield, while something like a gambeson or leather take into account the character having superior mobility (hence the very high Max Dex Bonus). DR/damage immunity were a property on rarer materials (like adamantine) and having them applied constantly is a very big deal (since you normally get them from temporary magical effects like protection from elements, or stoneskin - which is a very disappointing spell in 5e, in my opinion, compared to its previous incarnations).
Them wanting to streamline the hell out of their mechanics to purge the math that would scare the wider public away certainly is a good reason I went to look for solace in Pathfinder back in the day. The vulnerability/resistance/immunity triad hardly allows for any wiggle room for having subtler effects - like how every tiefling just takes 50% less damage from fire, rather than, say, absorbs 5 points of damage from fire attacks. You can't do anything more with that kind of DR apart from just getting full immunity.
