I often see teams go full AD but they still manage to win despite the armor stacking but I can't even remember when the last time I have seen a full AP comp win because of the MR stacking.
Is armor too useless or MR is overpowered?
armor is useless since you can have 200 armor and still die in 5 seconds. Ap generally isn't as powerful because their damage is not constant, they don't benefit from lifesteal, don't get crits on spells, require mana for spells. Only a few exceptions are like azir, teemo, kayle who can deal constant damage with their autos.
Because there are more AD champions that have true damage mechanics and sustained DPS that can shred through armour Eg. Yasuo sustain Riven with conqueror and ADCs. Also AD champs tend to be more snowbally because theres less armour early than there is MR, but armour per level is high, meaning if an AD champ gets a lead, they generally snowball harder than an AP champ
AP champs are generally more burst oriented, meaning if enemy team is allowed to just stack MR, you're making people that rely on oneshotting useless
Because AP is easier to build against. 
alone can fuck over almost any ap assassin. 
no ad champ cares about this item.
etcBecause ad gets more %pen, more true damage, more flat pen (lethality) and armor items are trash.
I'd like an answer to this as well.
Other way around mate. Maybe if all APs forget to buy Voidstaff, but with it it just isn't happening.
Because Armor is useless , and 1 MR item makes a mage useless
Because ap has less options. There are barely any top and jg ap champions and almost no ap adcs.
There is your answer. The problem is the champions.
Like do you not remember what happened last year since full ap was pretty common?
thinking about it...it's probably more to do with that with how things currently are (like a 50% ap ratio on towers), if you could build a full AP comp, it would be next to mandatory because it would have fewer down-sides (lots of range, decent engage/disengage, plenty of hybrid damage champs, etc)
the same is true in the past. It's really a miracle that the original developers created such an INCREDIBLY STABLE system, but things are getting out of hand
