Sarah Perez, writing for TechCrunch, tries to answer the question:
But perhaps most importantly is the fact that some number of Android owners aren’t downloading mobile applications at all. Google tacitly acknowledged this fact earlier this month, when it made a change to the way it measures Android version adoption on its Developers site. The company explained that, going forward, it would only show data reflecting those devices that had visited the Google Play Store.
Or in other words, there are enough Android devices out there which are not visiting the Google Play Store to affect the data that developers most care about – people who might download their apps.
This “Android engagement” conundrum has been discussed for many months now. A number of theories abound. Some believe there are quite a few Android owners who simply don’t use their phones like smartphones. IBM’s Black Friday 2012 data seems to back this up. These users are phone-first, and “smartphone” second.
I can’t speak to the breadth and depth of Android apps, but I’d say the primary reason for the iOS-first mentality is simple: money. The fact is developers have a much better chance at making significant dollars by concentrating solely on iOS, at least intially. That’s why it took so long for apps like Instagram and Instapaper to get Android versions, and why The Magazine will probably always be exclusive to iOS. Further, iOS has an advantage insofar that they have both a large install base and more opportunities at making big profits. Android may very well have more users, but by all accounts it seems much harder to really make a living there.