i think its the training and nutrition to be honest.
the training that most pros do today is absolutly stupid, they just go in the gym and wing it. to get the "pump" or "burn" or whatever dumbass thing they shoot for. If you started talking to any american pro about periodization or dynamic effort training he'd probably pass out from overthinking, either that or he'll start foaming at the mouth about some bogus weider principle. the thing is american pros can train pretty much any way and get by. (although you have an army of frustrated losers doing cutlers routine every week and not getting anywhere)
the eastern block bodybuilders probably borrowed a lot from the eastern block strength principles, the same principles which allowed their athletes dominate in olympic lifting for decades. they adapted those principles into a bodybuilding scheme and there you have it. scientific and intelligent training.
and i'm sure their nutrition is much better, from a micronutrient perspective american grown food is bankrupt.