The fundamental problem is that BBs outside of the Weider sphere of influence really didn't get much good exposure in the 1970s, either in monthly magazine coverage or unfortunately, in Pumping Iron, which unnecessarily departed from the more accurate depiction of BB in the book.
Even a few minutes of training footage of Oliva, Nubret, Robinson & Padilla for example, would've allowed them to be recorded on even footing with Weider's stars for posterity. It was decided to make the movie deliberately Weider-centric.
As a result, everyone has vivid footage of Schwarzenegger in top shape, but not anything comparable of Oliva or Nubret. Oliva in particular we're left with precious little quality photography or video from which to make fair comparisons.