He left due to personal reasons, illness. But let's be honest, he was never needed in the first place. What Nokia needed was a couple of first class technical officers that could put together and lead teams that would make Symbian and Maemo/MeGo rock. None of this happened (the only one of real CTO caliber left to HP because he couldn't stand MeeGo, a dead fish in the water)

When MS came into the picture, everything else became history.