I agree that the only reason shows should be withdrawn from are because of severe unresolved
safety concerns, severe illness or death of a family member. Once you commit to a job, you should stick to it.
Obviously in a more
amateur environment, people see these unwritten rules as optional, and walk if they don't like the director or SM, or because they'd prefer to hang with friends or party on particular nights. These people just have to be remembered as less committed individuals, and in my case I will not ask them to help again.
As mentioned by other posters, if you have a constant issue with people walking from you, and you only, you might want to revise the way you treat them in the working environment. You may be expecting them to do too much, treating them unfairly,
etc. Although this isn't a reason that they should walk for (instead talking it over, fixing things, or just living with it and noting down not to work with you next time), it's going to happen whether you like it or not.