He became unreliable. During and after his divorce, be started drinking more and more. The other guys had to convince him to go to rehab twice or thrice, I think. He also basically showed up drunk for a 2007 Unplugged show, I think. The final nail in the coffin for him was when he just didn't show up and didn't inform anyone. Jon's ego is too big to just go and ask Richie to come back after everything that's happened.