Think back to just a few years ago and Google's Android operating system may have actually had a smaller user base than that of Apple's iOS. That has since changed but the Play Store still lags behind the App store in terms of profitability. The Play Store at this point has already surpassed the App Store in sheer number of app downloads, a factor that is most likely due in large part to the higher number of Android users compare to those that use iOS devices now. Google and the Play Store are closing on Apple's profits from apps though, Google is bridging the gap and within just a few short years, analysts are predicting that Google and the Play Store will have surpassed Apple and the App's Store's profit numbers.
Freemium apps continue to be the driving force of app download numbers for Google, and reports from App Annie are showing that iOS users are starting to follow suit having downloaded and installed fewer paid apps so far this year compared to 2013. While Apple users still prefer paid apps more to those of the freemium nature, the fact that the use of freemium is growing on Apple and is trending to continue, will play a role in how Google and Android will overtake Apple's profit numbers to become the most profitable app store out there.
There is no debate that Google and Android have a larger marketshare, with over 80% of the market in emerging regions and it's for those reasons mostly that Google will be able to take the crown for most profitable app store in a few years. The larger user base of Android devices paired with fewer and fewer iOS usres choosing to pay for apps(with paid apps being Apple's bread and butter, and the reason they have been able to stay on top), there's nothing to stop Google and the Play Store from generating the most revenue from apps and games. Other factors play a role here too though as it's not just a numbers game with amount of users, that is simply the biggest reason. Android has more developers willing to port apps over from iOS and Android is seeing more content developed for the platform now that it's easier to develop for current and older versions of the OS compared to just a couple years ago.