Some of the reasons why I think some developers fail is because:
1. Lack of funding both the starting aspect and the launching aspect (lack of money to continue)
2. Failure in out righting frauds
3. Failure to evolve especially when the coin initially start bomming
Most developers have only one vision which is making money that is why many coins doesn't grow and they failed because they didn't see the vision ahead.
I think there are developers working on projects to build other projects that exist in the real world and they collect income. We remember that coins were published to help develop other projects, for example for online watching applications in the past, or developed for just the gambling world.