Alternate theory: there's a shit load of money to be made with legal marijuana so it will be mainstream just to ensure those paychecks keep coming. And for media, it's a low hanging trope that if you want your character to seen edgy but not dangerous, you have them do safe drugs like marijuana. It's easier than the weeks of story it would take to let the audience know they're just slightly anti establishment.

I'm not sure I buy your theory even a little, to be honest. The children of the 60's have, by and large, gotten FAR more conservative as they've gotten older.