vs 
all mr items are so much more expensive and not nessicarily better than armor items, you can stack armor and have it be more effective than stacking mr
an ap tank wins every time vs an ad tank because of base stats alone, and now they win even harder due to armor items being more effective
AD damage is more consistent vs AP damage which can happen in short or long bursts.
In other cases, AP damage over time has seen nerfs, and buffs, and nerfs, with buffs.
The way MR scales is different, too. 200MR =/= 200 Armor damage reduction.
It doesn't take mana to use Auto Attacks, but some Champions don't use Mana for AP damage.
There's a large complexity about it that just makes MR so needlessly complicated. I've never been a fan of cheap armor for APCs.
