Ben Johnson is one of the most famous cases of doping in sporting history. On the same way that Ben Johnson was caught cheating threre're so many other athletes who has been found guilty of the same sin that it seems to me quiet surprising that some supposed professional people on sport may fall allways on the same mistake.
Why does it happen so often?: Maybe they're too confidents on their good luck, or they're too confidents on their doctors, or they ignore at all what they take, the rules and it's just that they want to win and they're ready whatever it takes to win.
On the other side perhaps some sports are specially followed by the journalists. Cyclists, for example: It's unfair the outcry against these athletes. The authoryties should start to take control under the doctors, the trainers and the managers.
The most affected sports by doping are those who demand the bigger effort to the athletes although that the benned substances could'nt enhance no more that a few seconds. Just a small enhancement that would allow him to cross the winning line ahead to his competitors.