because they are easy to goof on and were kind of like the burning man of their day where rich kids would become flower children for a few hours listening to their favorite boomer jams. jerry garcia's ethos is quite known and it was very laissez faire towards everything in typical hippy fashion so it meant drugs were always round even if there were kids backstage (roadies and groupies brought families). deadheads as a whole come from all kinds of political backgrounds but generally are individualistic and view themselves as "free" or whatever from the "system" no matter what their politics is. very libertarian friendly in a way.
if you can put aside the cringy cultural stuff around it and deadhead culture, the music itself is incredible and as musicians they absolutely would kill it when on top of their game, but playing for 30 years every night people also hear their worst. once you listen and understand the band, the deadhead culture makes more sense. this video convinced me they were a good band https://www.youtube.com/watch?v=luAqu8VX5wo