Gee idk…maybe because he wants people to think he has great genetics?
I don't think it's so much this either. Both drugs and genetics take away from they want you to perceived and believe. That they are champions because they train harder, sacrifice more, are more disciplined, stricter with their diet, research and know more about nutritional supplements.... their success all comes from within -- their drive and determination.
For them to admit how much drugs they really used diminishes their hard work and sacrifice (and they do work hard and make a lot of sacrifice. Anybody that has dieted for a show knows one aspect of the insanity they put their body's through).
Not as much with genetics as it is offset by letting people think and know how special you are.
I remember as a kid I use to hear the phrase: "Champions are made not born."
It's really the opposite. In nearly every, if not all, endeavors, the best of the best are simply gifted. Tiger Woods was world class at 19 years old. How could he have possible trained harder than anybody else being so young. I remember when they were recruiting Le Bron James to the pros right out of high school. How could he have possibly been doing any more and better than all the other college athletes that have also been shooting hoops since childhood?
In the case of bodybuilding they are all juicing to the limit, carrying around their 8 meals in tupperware, in the gym... the reason why Coleman dominated was that he simply responded better than everybody else.