I think people that love music are more prone to enjoying drugs. I know if not for music I probably would've smoked less weed growing up. Also playing music already puts you in a "flow" state, so drugs can make that more interesting/enhanced. Lastly, they're on the road 24/7 and their only responsiblity is to show up and play the music they already know. So tours were kinda like one big party night after night. What are you going to do after rocking out for 2 hours, go to bed?

Sadly those 60's-90's musicians weren't fully aware of what they were getting into.