Many of you are citing famous bands. I'm going to interpret the thread differently, and share why I once left a productive band.
It was a 4 piece. I liked the BL and bass player very much. But the female lead singer played the Diva thing to the max. No help with anything. Late to every gig. But worst - very, very, Very drunk every gig. After a while, that gets pretty old for me.
But the real reason I left what was otherwise a very productive, frequently gigging band - the BL always wanted to take the dive bar gigs, not the upscale ones. We'd get nice restaurants offering us to play once a month, every month, but instead, he would bring us to these really nasty dive bars that were unattractive, small, and there for one and only one purpose - for everyone to get very, very Drunk. Eventually, I decided I was too old for that nonsense.
It was a painful decision because it lead to me being offline gig-wise for quite a while. But it all passed.