While bodybuilding isn't really my thing, my understanding from speaking with many new guys, is that they swear there is a certain gram range with Test that once you jump into, The gains and overall physique look just explode. I forget the range, but everyone swears it's night and day.
Also Test for many years has been a base drug, always cheap and easy to find, etc.... So that probably something to do with it. Just so easy to use piles of it.
Agreed, interesting question.