Hard work, unheard of drive, good eating. I would say that combined with learning to know their own bodies. They didn't say "oh f' some research article says only to train my body once a week" or "some strength coach says im doing it all wrong" they found what worked through trial and error, and kept at it.
Also it seems they cared more about the overall package. I notice many prefered to have athletic looking thighs with huge calves. that combined with the tiny waists made them look like statues come to life. Today you have guys who care about size over all, and will do anything to get it. Where back then they probably shyed away from taking too much, as they wanted to keep everything flowing together?
I'm just guessing, until Nelson gives us the answer!
On a side note, I often wonder just how much better off we are these days. With all these coaches telling bb's what to do, all the online gurus, all the scientific programs, etc... Maybe we all just screwed ourselves over the min. we started to listen to all those reports, and coaches. It's funny how many a time we will say people like Arnold, Robby, Zane, etc were all idiots and didn't have a clue, but then we will blindly follow the first guru or strength coach to come along.... Then when someone like Nelson or Sofageorge comes along, guys who were actually there back then, they too get written off as not knowing what they are talking about. Go figure...