My take on this , which came from my roomate in the USMC who was learning about mechanical stuff at the time , thus read everything he could on the subject.
Waaay back in the day , when GM was about to reveal their new push rod V8 , they made the decision to release all the specs on it to the after market , which allowed the aftermarket companies to build a performance part rather easily , without all the R&D required for Ford stuff , who was trying to keep their motor stuff "secret".
The end result , go fast parts were made by everyone for the SBC , and they all had to compete against each other , which drove costs down. Thus , the motor became very popular , which increased the market for it even further.
Add to that , the interchangability , its a hot rodders wet dream.
and then there is Ford , with about 27 different families of motors , none of which interchange with each other.
So tell me , if you were there back then , and building a hotrod and it HAD to go fast , and you had a small budget , what would you do ?