Why? Because the new advantage allowed him to pull off fast times without having to worry about sound mechanics, or because the doping creates physiological changes that bring about deterioted mechanics?
I don't think it was the latter. I think it's more likely that it was a case of 'better coaching through pharmacology'. That is, everything that he was taking covered up the fact that he was with someone who either coached or accepted very poor mechanics (there are other runners in the same group who are equally terrible) in their sprinters.