Apple XCode existed before the Apple iPhone. The SDK was necessary to support development on the iPhone and iPod Touch once it came out.

The API was so well-documented that it's extended to stuff like Unity3D, Visual Studio .NET, and other IDE's - which accelerated adoption.

That last part is what's wrong with Qt imho. It's been around for ages; however I can't just open up TextMate or XCode or Visual Studio or Komodo ActiveState or Coda and start coding off the bat.