I still remember one of the first HTC phones which had a mobile version of I think windows xp. The phone came with a stylus and the windows felt like a miniaturized version of desktop. This improved significantly in the Lumia series, the UI was good and easy to use. But by then the app ecosystem had already built strongly around android and ios so there was not much traction. I feel microsoft lost the ecosystem battle rather than OS. With too big app stores, the developers did not have much appetite for supporting another ecosystem which was very small.