If you click AP and armor you get zhonya's... while MR and AD, you get several options
EDIT: Well even Attack speed and MR... it feels like AD's have more options to stay alive while mages are required to get Zhonya's and then take items that detract from damage and go tanky while AD's can get several anti AP ADC items
because the champions who build AP aren't suppose to be resistance tanks, ideally you're thinking about battle mages.
click HP and AP and you'll notice a wealth of options.
To be fair, Zhonya's is an incredibly powerful and versatile item. You might not have many options for AP + Armor, but the option you do have and its build path are pretty spectacular.
If you click AD and armor, how many options are there?
Because Riot's biased towards AD and majority of players are AD mains.
Zhonyas is pretty busted honestly.
Because there are not many mage tanks. The few tanks that build AP typically combo with other tank items; 
to name a few. Zhonya's is also meant to counter AD assassins/burst for mages (their biggest opponents).
