There are so many reasons to leave a job. I can only say why I left jobs or rejected job offers in the past:

• Left a bullshit job. I was bored.
• Left a job because I didn't like where I had to live.
• Left a job because the company was unraveling. It went under within a year.
• Let a job because of incompetent management and crappy code.
• Rejected an offer because the place felt like a morgue.
• Rejected an offer because the hiring manager's boss acted like a entitled asshole.
• Rejected an offer because the work spaces for developers were even worse than open plan.