Its because developing for the iphon is easier than android. Think about it. Every iphone is the same. They have the same hardware, same software, same size screen etc. All Android phones are different and can be different in many ways. From the handset spec to the version of the OS and then you have to worry about thinks like HTC Sense etc.
If you develop an app for the iphone you know it will work on every single iphone. If you develop for Android you cannot be sure that it will on every Android phone out there.