From the Flurry Blog:
Anecdotally, developers consistently tell us that they make more money on iOS, about three to four times as much. To be sure, we pulled a sample of in-app purchase data from a set of top apps with versions on both iOS and Android, comprising of several million daily active users (DAUs). Running the numbers, we find that, on average, for every $1.00 generated on iOS, the same app will generate $0.24 on Android.
Developers follow the money. Duh.