If that was true why wasn't Zane on more covers than Arnold?
Good question, good sir.
I can only go based on what Peter McGough said in that documentary on Joe Weider that may have come out around 2010. I assume some "politics" are at play in covers - back in the day, FLEX had a sort of monopoly on hardcore bodybuilding magazines. To be fair, it was not a monopoly, but those of us old enough remember what it was like before the internet - you had to rely on magazines, and the pickings were slim. I remember buying a FLEX magazine around two months after the Olympia, and it was normally only by that point that the results made it to print. By then, the results had been posted online, as I started following bodybuilding in 2000, but by the time of FLEX's inaugural issue in 1983, you really had very little outside of the magazines to depend on.
So under that framework, one could say that Joe Weider had a bit more leeway to direct the magazines as he saw fit, and so if Arnold was Weider's favourite [as has been said many times], then it stands to reason that Weider could have put Arnold on as many covers as he wanted.
Compare that to the comments that Shawn Perrine [RIP] made in a VICE article where Dana Linn Bailey was complaining that the judges were oppressing her for not having implants. Shawn stated:
"We want to feature top competitors [on the cover] and it just so happens that they have breast implants. It's all market-driven. Whether it's breast augmentation or a certain hairstyle, everything is market-driven. If that's part of the formula [that we use for the] cover, and it sells, then that's what we are going to go for.
https://sports.vice.com/en_us/article/pg55vg/the-judges-want-to-see-boobs-how-the-competitive-fitness-industry-pressures-women-into-getting-breast-implantsApparently, the billion dollars anti-capitalist organization VICE took issue with AMI using a market-driven approach.
But basically what I mean is that these days, magazines have less leeway to play favourites - if they want to survive, they need to sell to what little audience they capture, so presumably a complete market-driven approach would be the way to go.
Your post motivated me to do a direct comparison of the respective cover histories from
www.musclememory.com of Arnold Schwarzenegger and Frank Zane prior to 1990. I didn't include the covers from 1990 onward because those were banking on Arnold's Hollywood star power [from 1990 to 2003, when Arnold entered politics] or nostalgic covers, also selling Arnold's popularity [2003 to 2012].
As you said, Arnold did have more covers than Frank - but it was remarkably not as far off as one might guess. So apparently Frank did sell magazines, and his vast cover list goes to show that.