Nokia was too invested in Symbian, which was ill-prepared for the new world of rich smartphone experiences. As a much older platform, it had been architected around some faulty fundamental assumptions, like the phone processor only having a single core. Nokia also had an army of middle management internally which had built their careers on Symbian and would have lead an internal revolution had Nokia pivoted to Android back when it was opportune to do so.
Nokia didn't go with Windows Mobile because it thought it was the superior platform. It went with a different platform because it had become painfully clear to said middle management that Symbian could not be economically technically adapted for modern smartphone hardware, that sales were tanking as a result. And then Nokia went with Microsoft instead of Android because Microsoft gave them a boat load of money to do so, whereas with Android, Nokia would've had to build everything from scratch, and it wasn't clear anymore that they'd have the resources to do so.
Nokia's story is, more than anything else, of how technical debt can kill a company.