I'm not saying this is the exact reason and it might not have anything to do with it, but I'm sure all those years of drug use didn't exactly do the band any good. They say drugs can take off years of your life.
Man, cancer is really getting bad in music recently SR-71's first bassist, recently died from cancer ("In My Mind" by SR-71, one of my favorite songs, was written about him), Simplistic's guitarist got hit, and a lot more... geez.
You know that the world is screwed up when Joey and DeeDee Ramone and Joe Strummer are dead, but Johnny Rotten is still alive. But according to this, Johnny's not going anywhere too soon. SOURCE