I'm picking this up a bit late, two years late, but thought it worth mentioning a couple of reasons I have not seen expressed in this list, not in the same words anyway. I'm curious, because I'm about to leave my current band for these reasons, and want to check that it's not me being the control freak a-hole others have mentioned as their reason for leaving...
1) The band has clearly demonstrated its level of ambition, and it's too low for my liking
Over the years I've come to believe that "commitment is as commitment does", by which I mean, if the guys are all dead keen after a gig and talking about the big things we can achieve, but don't answer your communications trying to organise the next rehearsal, you can write off the keen chat as just adrenalin rush BS. We played an error-filled gig in front of a very supportive "home crowd" in their local pub, and they were strutting about like they'd just won an Emmy. Nobody's exactly saying we've arrived and don't need to do any more hard work, but there's a clear lack of urgency about rehearsals, fixing errors and new material.
2) The band have fallen into the role of children, with one member acting as the parent
One of the things I felt I could bring to the band over and above my playing was a bit of professionalism and organisation. I hoped it would raise individuals' standards a bit, but it has had the opposite effect: People don't bother taking notes, because they know I will (for which reason I've now stopped printing out extra copies in anticipation); Coming up to our biggest and longest gig ever, I was the only person able to do the simple sum that says "we need five more songs"; Same gig, two of them announce - when I double-check the day before - that they don't have working amps; Gig in their local pub (I live 50 miles away), but I'm the one figuring out what kit they have in the pub and what we'll need; Trying to confirm a gig booking, in that anxious time between the offer of the gig and confirmation back that we can all do it, nobody answers emails or texts, and when I complain, I'm told "if I don't reply it means I can do it".
The last straw was in our most recent gig, two songs to go and building up to a nice climax, not one but two of them announce that they need to go to the toilet. It was when I was adding to my mental gig checklist "make sure everyone goes to the toilet during the break" that I realised things had gone a bit too far.
Seeking solace from family and friends, I've been advised "They've got too used to you rescuing them. Just let them fall next time", but it's hard to find a way of letting them fall that won't result in me being embarrassed as least as much as them.
I'm curious as to whether anyone has experienced 1 or 2 above. I put them both in the Incurable category.
A word on substance abuse: In my own experience with bands, the sense of raised awareness and being right in the groove is limited to the person who's taken the drugs. Everyone else realises they're playing the wrong song.