It was the lack of vision. They kept doubling down on Symbian for 4 years thinking they can make it competitive with iOS and Android. It was the wrong decision.
They should've kept supporting Symbian, but focus on Maemo (or a new OS) from day one (after the iPhone launched). Supporting WP7 over Android will probably be the final biggest mistake that Nokia did.
Classic innovator's dilemma: The management made all the right decisions to avoid cannibalizing their own Symbian revenue. They just didn't have the courage to take the steps necessary to bet on the future.
They should've kept supporting Symbian, but focus on Maemo (or a new OS) from day one (after the iPhone launched). Supporting WP7 over Android will probably be the final biggest mistake that Nokia did.