Why devs choose Android
Some developers have personal principles that demand open source, and they would choose Android based on that. I doubt that's a huge number though.
Certainly the audience penetration of Android is far greater, but Apple's advantage in revenue from the App Store versus the Play Store actually increased last year, from 70% in 2014 to 75% in 2015! There are several obvious reasons why developers earn more from the App Store than they do from the Play Store despite Android's huge market share advantage, but it comes down to the fact that while the Play Store has twice as many app downloads as the App Store, developers will still make more money developing for iOS. I suspect most of them still write apps for iOS first, since that's still where the money is.
The remaining growth in the size of the Android market will all be at the extremely low end, so even once feature phones are gone entirely these numbers are unlikely to change much. Unless you think people will start abandoning iOS for Android in greater numbers than those abandoning Android for iOS, developers will continue to make more money from the App Store.