Not a surprise really...developers realized long ago that ad supported apps made close to nothing on Android vs. iOS. I cannot say what the main reasons are really...but after a failed early attempt to make more than $50/month on an ad-supported android app that had the same number of users as my $1000/month iOS ad supported app...we quickly realized it was not financially sound to continue to support android.
This is definitely not good long-term for android because developers need to make money one way or another off apps...otherwise, there is no incentive to build them...
My guess...higher quality apps will continue to come more consistently to iOS vs. Android...I'm not sure this matters really because Android users tend not to download or consume apps very much...
I'm blown away that someone is paying you $1000 / mo to show ads in your app. I figured that was reserved for big whig corps only and it was hardly worth it to do an ad supported app in iOS, much less android. How many users is that? Good for you BTW. The only advertising I'd pay for are ads that Dont need to be clicked to do their job, like new movie releases or branding. Im really blown away by the fact that people actually click on ads. No wonder malware is so rampant on windows.