Frank took the sport into a new direction. Quality over quantity!
Actually Weider took the "sport" in another direction, Zane was along for the vehicle at the time much as Schwarzenegger was 10 years earlier over Oliva. The parallels are obvious, given Weider's desire to increase sales and prominence.
Very much a no-brainer: anoint marketable, articulate caucasians Schwarzenegger & Zane for a string of victories and concurrent increases in interest and sales, or allow questionable semi-english literate non-whites to win, who wouldn't move his magazines & supplements. Very very simple from a business perspective, and Weider was always about the bottom line.
When Zane won, health & fitness was on the upswing in the United States. Greats like Robinson, Fox, Szkalak, Mentzer, & Padilla weren't given fair opportunities and for the most part all drew the same conclusions and left.