You'll need to contact the developers directly for an exact answer however my guess would be to do with install base. Android has a huge install base allowing companies to make a hefty chunk of change on that platform. Windows Phone has a very minute install base by comparison and therefore money from ads will not be enough for the company to stay alive in most cases. Not only that, but the nature of Android is that everything has to be free (therefore making you the product by use of ads) and thus a lot of apps go by this mantra, whereas Windows Phone users are generally more willing to pay for their apps.