I just don't think he had the drive to do it anymore. When he came in the HW division was in the down swing and he was able to just come in there and overpower guys with his strength and wrestling background alone. The division got MUCH stronger since then and he just hasn't worked on his game, his striking and more importantly how to respond to taking a hit just never got better over the years. Plus his health issues took a major toll on him. It's a blow for the UFC but they'll move on the sport is in it's boom period and they have enough interesting and exciting fighter to carry them forward.