This is the way I see it, AP Teemo can only go so far. Your only two nukes are your blinding dart and your mushrooms. You go in and blind someone and you're somewhat forced to run away for 5-8 seconds while your blind is on cooldown and your E has horrible scaling. If they do chase you and run into a single shroom, they probably will retreat after that. As for shrooms, once the other team buys oracles, your shrooms become less effective. In a teamfight situation, Teemo's short range will be the death of him since he's so squishy if you build pure AP with no form of defense. Also with the meta being tanky DPS meta, the shrooms just won't be hurting too much. I love playing teemo, and I used to play him AP, but unless you can end the game early, you slowly start to become less and less effective. AD teemo on the other hand with items such as wriggles, frozen mallet, and madreds and the E. You can basically chunk their health forever while keeping yourself sustained and maintain a good amount of bulkiness. Black Cleaver, Madreds will tear anyone a part
tl;dr. AP teemo loses effectiveness late game, AD teemo doesn't.