the problem with ap/armor items is, that mages arent supposed to have too much access to defenses.
they have a massive range and utility advatage. imagine you couldnt even fight them when you catch them out in melee, because theyre too tanky to take down before they cc you and disengage.
its already difficult enough with certain champions like 
the moment she shields herself, she becomes tanky as fuck and really hard to take down. considering how much of a pain it is to already get into melee range of her, if she was running around with 2 ap armor items (one of them being zonyas) on top of that would be horrendous.
mages also have a lot more access to ap/health items than they used to that would synergize too well with the additional armor options.
i already hate the fact that theyre giving marksmen way too much access to defense on their offensive items, and would appreciate if they removed a couple of these mechanics. lets not add mages to the problem.
certain champions are just meant to be squishy and to die easily when they mistep to balance them out.