Currently Teemo is extremely week in the sense that control wards along with red trinket both disable his shrooms. QSS and every spellshield also stop/cleanse. Why is he still disabled?
Now if only they removed him in other game modes too...
Mostly because Tunnel Snakes rule 
he's the lil devil. thats why
Same reason as Karthus.
Because it would be stupid boring to play against. Ive afked in base vs ONE Teemo because he had so many f---ing shrooms in my own jungle I couldnt leave my base. Now multiply that by 5. And one red trinket with a massive cd doesnt make Teemo "useless". It no longer becomes LoL, it becomes "hunt the shrooms" and its so time consuming and retarded to deal with nobody wants to play vs that. I would literally just afk from the beginning if the other team had Teemos.
The real reason is PC strain. OMG if 10 mushrooms go off on lower end pcs they would go to like 5fps.
It's because 5 teemos at level 6 can just put all their shrooms down each lane behind a tower and keep replenishing and wait. The game would be boring as fuck for the enemy team, not to mention the teemo team, sitting there waiting for people to come down the lane who aren't stupid enough to do it after they die once.
It's dumb man, no gameplay. That's why.
