You have to make bodybuilding contest more then flexing in a thong. Make the competitors run a mile, have a bench/squat/dead compition.
Bodybuilding contests used to be the opening act, so to speak, for weightlifting meets, back in the early days of the AAU. Eventually, the bodybuilding portion became more popular than the lifting part, as being a weightlifter did not necessarily require a great-looking physique.
Unfortunately, bodybuilding has three major image problems:
1) The participants don't do anything but pose. If people want to see big muscular guys, they can do so by watching RAW and SMACKDOWN. Before I got into bodybuilding, I'd see guys with muscular physiques by watching wrestling every week.
2) The steroid use: It's not just that the competitors use anabolics; it's the perception that such use (and heavy use at that) is virtually mandatory. People are, by and large, shocked when folks from mainstream sports are said to use steroids; that ain't the case with bodybuilding.
3) Image problem: Outside of the anabolics issue, I think the biggest problem in bodybuilding is that people still see bodybuilders as selfish, narcisstic, egotisitcal, and stupid jerks. It certainly doesn't help when you have a certain supplement company feeding into that stereotype, just to sell its goodies (No, you do NOT have to ignore your wife/girlfriend, live in a rat-shack, look down on people who aren't into bodybuilding, and spend your rent money on supplements to be a champion bodybuilder).
Bodybuilding won't become mainstream, until it gets a major fan base comprised of people who DO NOT work out. The majority of football fans have never put on pads, cleats, or a helmet.