I think it always comes down to the apps.

Windows phone died because of its lack of apps. Same thing with several other mobile OS's. Ubuntu has a really great OS and UI, but no apps for just basic things renders it useless to even the most basic of users like myself. I don't have games, no banking apps, a few email and Microsoft apps and yet I still couldn't find a way to make it work.

One of the other technical limitations is network. Ubuntu has yet to solve the VoLTE (Voice over LTE) riddle. This is a major sticking point for US consumers.