The line has become blurrier in recent years, but the *openness* of a platform plays a large part. iOS, Android, Windows Phone and other phone and tablet OSes tend to be fairly closed platforms that don't give much control to the user and can't be easily used for general-purpose computing. That said, yes, all these areas are converging. We had a [discussion on tablets ~2012](https://meta.superuser.com/questions/5045/please-vote-on-proposed-changes-to-our-policy-on-tablet-computers). 

AFAIK, phones are still officially off-topic. If you truly believe that should change, you're free to ask another meta question proposing the change.

See also: https://meta.superuser.com/questions/4885/how-are-we-going-to-deal-with-windows-8-on-arm (no complete consensus there)