So right away, if you want a successful smartphone app in the U.S. then you need to target, urban white men between 18-29 with at least some college and 75000+ a year in income.
I think as you say more people should target international etc... but people build toward the problems they have and the language they know. Whatsapp is killing it because they broke out of the mold. So yes, they should for many reasons, but if you just follow the money then who you target looks very homogenous.
[1] http://www.pewinternet.org/fact-sheets/mobile-technology-fac...
[2] http://www.voxburner.com/publications/6766-who-makes-in-app-...