Echoing @tez and @tat, when I leave a game for good (in those cases where I am still invested in the characters I'm playing) it has almost always been due to
- staff malfeasance or behaviours that I find toxic or unethical and
- being utterly unable to then get away from those people and just do my own thing in a corner with people whose RP I still enjoy, playing characters I still love.
There have been games where I haven't become invested due to any number of other reasons (sterile atmosphere, obnoxious players, setting didn't click for me). But when I love a game and leave it anyway... it always comes down to no longer feeling comfortable condoning the game and/or staff by remaining there. Once it gets to that point, the bridge is burned for me.