A few reasons:
First of all, the accusation that they never went off is dead wrong. In the off season Arnold wasn't all that amazing. Zane blew him away.It was Arnold's incredable ability to transform himself that was a part of his greatness.
But the main reasons are this:
There was a genetic elite, just as there is today, but there were a lot less people doing it. If Ronnie Coleman was 30 years old in 1969 and did 200 mgs of deca and 25 mgs of d-bol. he's look comparable to the champs of the day.
Also, it doesn't tke a ton of gear to look good. But we've become so acuatomed to the dosages that pros use in oder to hover around 300 pounds that the higher dosages aren't as shocking. But nobody here is a pro and using anywhere close to that much gear is as pointless as using a $100,000.00 set of clubs to play miniature golf.
And finally, the old timers simply knew more about bodybuilding. Yeah, I know, we'd like to think we know so much more but the truth is today, we have a lot more WRONG information. Back then, those guys got into it for health purposes. The built solid foundations. They experimented with different concepts and techniques. Their approach was artistic and analytical. Bodybuilding was a cult activity and the guys involved felt thay had something to prove in terms of the benefits. Today, more people are into it because it's a fasionable trend.
These days, somebody joins a gym, hooks up a source, trains like shit, does more gear, then goes on a message board and gripes that he isn't growing.
Even the pros are guys who just have great genetics and respond well to the sauce. But look at them. It's like they came off an assembly line. That much juice distorts your natural lines. Arnold, Scott, Zane, Oliva, Reeves, Draper...their bodies had personalities. Today, I honestly can't tell one from the other. With a few exeptions, they all have huge asses and disended guts.
Even if somebody looked as good as Arnold or Zane today , it simply wouldnt be as impressive as it was 30 years ago because we've seen too many people so much bigger.
And that, is the diffrence between then and now. And there's no going back.