Lots of promise but that's all it ever had
The biggest issue Microsoft have (had?) is that they were treating the phone OS as they would a desktop OS. I don't mean the app compatibility thing I am referring to the glacial pace of "one update a year". When Android and Apple have obvious (to smartphone users) features like copy and paste, multitasking and others it took Microsoft way too long to implement those features. Add to that their incredibly bad and slow app approval process and you have an OS that is so far behind the curse that it was hoping to rely on apps but their approval time stopped that happening.
I said at the start with Windows Phone 7, and again with 8, that it had a lot of potential but Microsoft did not seem to be very interested in putting the resources behind it to give it the massive shove it needed. The whole "smoked by Windows Phone" fiasco was pathetic. It might boot up and do something faster than my Android phone but how often do you turn off your phone? That was also using the horrible Windows Phone 7 which couldn't be upgraded to 8. Good plan! Brand new phone OS, brand new ecosystem, trying to push it as an alternative to Android and Apple - abandoned / no upgrade path to 8 a year later.
It's got to go down as a huge fail for how often they just did not get what people wanted and kept trying to tell them what they wanted - just like Windows 8.