We can conjecture all we'd like, but Sam Quinones wrote a rather definitive history of the opioid epidemic in America. https://www.amazon.com/Dreamland-True-Americas-Opiate-Epidem...

Sam ran the podcast circuit awhile back if you want to listen to his story. I found out about it via Russ Roberts' Econ Talk. http://www.econtalk.org/archives/2017/01/sam_quinones_on.htm...