there were a lot of problems. One that hasn't been mentioned is that MS's idea of Windows Phone was that all the apps would match the OS's design. A lot of big companies did not want to do that.
As a user I have to say I did want that. Windows Phone peaked with WP8 though, IMHO. W10M has always been weirdly buggy, inconsistent and not performing as well.
I guess the two API and forced hardware changes did most to cement its demise.