And Instagram's gradual iOS-to-Android transition is not unusual. It took almost a year for to make its pig-hating way from iOS to Android, and twice that long for . So it's clearly not uncommon for hugely successful apps to launch on the iPhone, only to show up on Android many moons later. But for apps to make the opposite leap--from Android to iPhone--is exceedingly uncommon. There are two key reasons why that dichotomy exists: money and simplicity.

In fact, if an app isn't built by Google, odds are good that it's coming to Android only well after it arrives on the iPhone--if at all. Sure, Android phones got Gmail first (and, frankly, ). But it's rare for Android to get major apps first, and the situation hasn't improved despite Android's continued impressive adoption rates.

Although recent reports suggest that , comScore still says that

But even if Android is the market share king, developers aren't targeting the platform with the enthusiasm they lavish on iOS. That's partly because of the other major devices that the App Store caters to--the iPad and the iPod touch--two lines that few Android devices can reasonably claim to compete with successfully.