Bodybuildings popularity(from Pumping Iron days till the early 90's) declined because
the lie was exposed. Through a combination of lies and the big Weider promotion machine, it was believed that with hard work, the right genetics and enough Weider protein powder, you could attain a physique like Zane's or Franco's or maybe even Arnold's
Once the truth was revealed the average guy who may have persued bodybuilding lost interest.
Even more people lost interest once guys started passing out on stage and frying their kidneys with diuretics.
I'm still a fan and have a tremendous amount of respect for guys who compete and push themselves but the golden years are GONE and they will never return
Oh and Nobody wants to see swimmers posing on stage either so the "Natural" route is a waste of time/effort