I use skritter.com for practising Chinese. They recently released an iOS app after several months work. There is currently no plan to produce an Android app, partly because there is a partial web / flash solution available on Android, but I assume also that the numbers didn't add up for putting the work in to porting the iPhone app.

This is a must-have application for learning to write Chinese in my opinion, to the extent that people would buy an iOS device just to use it. There is no equivalent for Android. I carry around an iPhone as a work phone, and pretty much only use it for this.

So learning Chinese is a little niche, but I'm sure there are other niches where people rely on a particular app, this app is not available on Android, and so they wouldn't switch for this reason.