I wonder who was running the PR for cardio and weightlifting in the 70-90's. For some reason weight lifting, and barbell training in particular, has a weird anti-intellectual connotation. Getting up to run for two hours at 4 am is the habit of a successful business executive, but spend a couple of hours a day in the gym and carry an excess of muscle mass and you're clearly dumb. It's very strange.
The Jim Fixx book "The Complete Book of Running" was a national best seller starting in 1977, then aerobics (studio exercise, a la Jane Fonda) hit, followed by Arthur Jones' Nautilus in 1986.
Free weights were considered "dangerous" for a long time. Frankly, most people who lifted back then were suspected of being homosexual.
If you take boomers as being born between 1946 and 1964, then the leading edge of the boomers were turning 30 about the time both Fixx's book and "Pumping Iron" hit the popular conscious. There is a lot more money in selling shoes to people than gym memberships, so saturation advertising is part of the 'why' as well.