Question for Riot on Armor Items?

Just a quick question, hope a red would be willing to share on the subject.

Why are all armor items tied to Hp or Mana? I'm not asking for more, I was just curious as to your reasoning for having only the exceptions to this in the form of Thornmail, Hourglass, GA, and zz'rot. I'm certain this is a deliberate design decision and not just happenstance, and just wanted to hear your thoughts. I get the idea of offensive + defensive stats on items tending to make them really snowbally, but that didn't stop you guys from adding a plethora of Hp/AD items. With AP being removed from sheen/IBG the only AP items with armor are zeke's and hourglass, and there aren't any items that give AD + Armor after the removal of Atma's. Not having more Armor + MR items makes sense because I don't think anyone wants tanks walking around with 250 in both resists.

I'm not saying there are no damaging armor items beacause Deadmans, IBG, Sunfire, Thorn etc are decent damage items for tanks looking to do a bit more damage. Why nothing for fighters, assassins, or mages that want to get destroyed slightly slower by AD while not trading up too much damage?

AP/MR items: AD/MR: (I count wit's end here) AP/Armor: AD/Armor: