I think quite a bit of it is boredom. There aren't very many people in bluegrass music with drug problems. Why? Because bluegrass musicians don't have luxury private jets or an entourage that's only slightly smaller than the population of Rhode Island. Many bluegrass acts tour as old country acts did -- in their own vehicles. When you're not only the musician you're also the t-shirt salesman, the sound technician, the roadie and the driver you don't have a lot of "down time." In the world of rock and roll, Elton John (who survived all these demons himself, thankfully) said it this way in the 1971 song "Holiday Inn:" "Boredom's a pastime that one soon acquired, where you get to the stage when you're not even tired, clicking your heels till the time comes around to pack up your bags and head out of town." Most of the modern acts have to "gear up" for two hours on stage, leaving them 22 hours out of a day with nothing else to do.
Another thing that might be an issue is that so many of these superstars don't have real contact with their fans, a "grounding in reality." Again using the bluegrass analogy, almost every bluegrass act, no matter how big they are, feels an obligation to (as Dailey & Vincent put it) "shake and howdy" with their fans. A lot of older country stars know many (and I'm talking hundreds, if not thousands) of their fans by name. In the world of rock and roll, most of these people have enough bodyguards to take out the Green Bay Packers front line, and you have to know somebody to get near them. The people who DO get in the "inner circle" of these acts want to stay there, so they basically become boot-lickers, or "yes men." People are afraid if they speak up when the celebrity is doing something they KNOW is wrong or injurious to the celebrity's health they will be "out" of the inner circle, and they don't want to lose that connection. Joe Walsh actually had to hire a "NO man" to help keep him sober!
Finally, the sad reality is that we, as a society, has made what we call "the rock and roll lifestyle" glamorous, almost to the point of condoning the notion that "early death by excess" is a good thing. We need to stop "celebrating" bad behavior. People make fun of the Osmonds for their "squeaky clean" lifestyle, but at least they're all still alive.