The reason for that is (This is just my guess) there are people who use speedy pokemon as stat builders. They will have pokemon with moves like "Swords Dance", "Acid Armor" Etc. and 1 or 2 Mega powerful pokemon to act as a Sweeper. The speed pokemon come out first, raise the stats needed for the tank to work, use Baton Pass, and then switch to the next stat builder. They continue this process with the stat builders until they're ready to bring out the tank. That is more than likely why the speedy pokemon are OU. The Speed pokemon raise Speed, Attack, Defense, ETC for the tank, and it's all downhill for the opponent from there.