It makes sense. Bodybuilding as an industry was only in the earliest of phases of growth- it's a privately run, profitable industry, so you do the most you can to make sure it's as profitable as possible. What does this mean? Better marketing- and of course, that means Arnold as the figurehead.
I personally don't think Arnold was any more marketable than Sergio , you have a Austrian with a heavy German accent or a Cuban with a heavy Spanish one , I don't buy either one being more marketable than a very clean cut college educated Frank Zane with a build that appeared attainable , or the tall blond blue eyed , epitome of muscle beach Dave Draper
Joe Weider did say if he put Arnold on the cover of his magazine he would sell more , but people make the leap that he fixed contests in order to do so which is a stretch , Arnold was on the magazines long before he started winning and if he wanted to fix the contests to promote Arnold , he would have never lost to Frank Zane in 68 or Sergio in 69 , it just defies logic
Lee Haney was Mr Olympia in the 1980s yet he didn't appear on the cover of Flex or Muscle & Fitness as much as Shawn Ray , who was ' marketable '