I think age has a lot to do with it, so does stardom. Blues just kinda get better as people grow up and grow old, but the really good metal seems to be done by people in the late teens through the twenties, by the time your in your thirties it seems harder to do. Stardom does not help, you get fat, lazy, too many fancy dinners, living large - hard to summon up the angst and energy to do proper metal. I still see some kick ass metal, but its the kids not the big stars doing it.