Mostly politics, I guess?
The CTO was about to be fired, so I had nobody to fight for me. The Chief of Product didn't like the optics of a non-PM being more successful than a PM in delivering work.
(EDIT: As I said, I delivered from start-to-finish 3 projects that other teams had considered "impossible", mostly because of bad specifications and overengineering, often caused by PM miscommunication. The PMs of other teams REALLY took the blame here: one was fired and sued by the company before I joined, and the other confessed to me he was "asked to leave")
As for why I don't change careers, I'm an Engineering Manager and I made twice as much money than a senior PM at my previous organization.
I still long for a position where I can be both technical, product and business minded. But I guess the only job where I can do that is as a founder.