A reason that is mentioned here often (possibly simply for derogatory purposes) is that the band doesn't want to go back to working at Target.
But really, that is pretty much the truth, isn't it. Classical music isn't about touring, fame, drugs, promised sex etc etc
A band makes a good album, like Blessed are the Sick, gets recognition for it, makes another album, but this time courts a little bit of popular attention with populist music, and gets even more recognition. Good job.
BUt then they find themselves at a point where they see that they are making no money on this music and this heavy life of vice needs fuel. Their music does not selll enough to ultimately help them make a living, at least not serving their sensual needs as well, so they look at it this way: "I did something great, and now I can keep doing it, but in a more popular way, and I can make an actual living off of it."
Why should artists have day jobs? Seems reasonable, yet most of the better albums out there are made by people who are broke and still have jobs that high schoolers would be embarrassed by and bad albums are made by people who decided the life was more important than the art.
Maybe that's why the best metal was made by someone who never toured, never lived the life, and in fact spent his later years in prison, until now.