Games dominate the mobile app ecosystem. Seriously.
It's easy to forget that as we all search for the next big, hype-worthy app of the moment, but it's the truth. Eight out of the ten most purchased apps last year were games. Nine of the ten highest grossing apps in the App Store last year, all games.
Combine this with the fact that Apple's Game Center was launched almost 2.5 years ago, and it's a bit strange that Google has yet to launch a Game Center-esque hub of their own for Android.