For me, it's because the brand name is poison. I wouldn't even look at Windows Mobile when it came out, after the horrible Windows CE devices and ugly desktop experiences I've had. By the time I realized that Windows Phone is a pretty decent platform, I was already well established with Android in my household.
The think that gave Microsoft power, that people tend to stick with one platform once they've invested time and money into it, is now working against them. It's pretty tough to get people to switch from iOS and Android. All your email, messaging, apps, etc are tied to a particular phone OS, and your phone is typically tied to a 2 year contract.
No idea how MS can pull out of this one. I've mostly chalked it up that the sun is setting on the Microsoft empire, and someone new is going to take over the 2020's. Probably Google, but I don't really know for sure. (crystal ball is in the shop)