Nadella, Gates, and Ballmer have all admitted to Microsoft’s mobile mistakes.
Which time?
Windows phone was the best phone OS I ever experienced. Features were years ahead of iOS and Android.
Windows Phone failed because there were no apps for it. There was no YouTube app, no Facebook app, no Twitter app, etc until very late or never at all. They should have just paid developers to make the apps so that people would buy the phones. The OS was great and worked on a wide range of hardware. It could have been a great enterprise solution and they seemed to be heading that direction but the lack of third party made it little more than A Microsoft feature phone.
That is because every single mobile version of Windows was incompatible (After version 6) with the previous. They kept reinventing the wheel over and over again.