Think about yourself. I'll bet it was more than the money. In a worker-centric era like this one, talented people will not work long for a bad boss. And, by the way, bad bosses come in all sizes and shapes. There are the uninterested, unskilled, unaware, self-focused and abusive varieties.
Most employees know that it can be 'career-limiting' to give the boss that kind of news.
Or, ask a friend at work to tell you the truth about your strengths and weaknesses. Now, if you have no friends at work, there's a clue for your clue-bag. Once bad bosses know what needs changing, they may need help from a coach or a mentor. And they will definitely need continuous feedback as they try on new behaviors. Highly motivated bosses will successfully change and others will do better to step out of the boss role altogether.