Was it you? Are you overbearing? controlling? Did you incessantly email, call, or text them? Did you insult them, perhaps even without knowing it? Did you give them an endless list of requirements? Did you give them a list of bugs without fail cases to reproduce? (Many developers will not debug something without steps to reproduce the issue).
Was the pay sub-par for the work being done? Was there an equity promise that wasn't fulfilled? Was there something on YOUR part that you didn't do that you said you would?
Many developers are introverts and will NOT get in someone's face about things that should have been done. They will just walk away. If you get a good developer, it's going to fall on you, the BOSS, to make sure you follow through on your promises and keep them happy.
Cheers!