I suspect the app ecosystem was a problem with Windows Phone. iOS and Android already had a head start of a few years, with Windows Phone not appearing until late 2010, and "Windows" was a bit of a misnomer, because desktop Windows apps couldn't run on the phone, so the preexisting software ecosystem didn't help.

Certainly the app ecosystem was part of the challenge, and Microsoft spent a fair bit of effort trying to both encourage developers to make apps, and filling obvious gaps (like Youtube) itself. If their resources, retail connections and brand recognition weren't enough, it's hard to imagine that anyone else stands much chance until conditions change drastically.