The rumor is that he left because he refused to sign a document that Apple Maps was a failure:
“Apple asked Scott Forstall to leave the company because Forstall refused to sign his name on a letter apologizing for Apple’s Maps app, according to a report. Citing “people familiar with the matter,” The Wall Street Journal reports that Forstall (pictured), a 15-year veteran at Apple, wanted the company to address the poor reception for Maps without apologizing. However, CEO Tim Cook disagreed and sent an e-mail to customers on Sept. 28 stating that Maps “fell short.”” – Source
I think the main question is why did Apple even try to compete with Google Maps? Google has invested so many millions into making their apps perfect. Why did Apple care to compete? Was there really more money to be made by having their own maps? The reason I ask this is because a large % of Apple users had no idea Google made their maps and still has no idea that Apple now makes them. They really dont care. They just want maps and gps. Apple should have never tried to compete with Google on the maps app unless Apple was planning to do something innovative with the product…like what they did when iPhone was first invented. Sure their maps is going to get better and get good but Google maps is really good and is getting better too. Why they chose to compete with that app is beyond me. So here is my message for Apple: Do not compete for the sake of competing and having a monopoly. You will never be better than everyone at everything. Let this be a lesson learned. If you cannot improve something why try to recreate the wheel? Unless you can make that wheel better do not waste your time and energy on it. Why not invest in making iTunes user friendly? It has to be the worst piece of software I have used (from an HCI perspective that is). I could think of a 20 things off the top of my head that Apple would have been better off investing in or improving. Why they thought Apple Maps was a good idea I will never know.