Here's something to shed some light on Bungie if they think its because of small ecosystem.
https://medium.com/@tjesboogie/heres-how-many-downloads-you-get-on-windows-phone-c4395cc96104
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1,500 downloads a week is nothing for the iOS App Store though. There are apps that do 50,000 downloads a week and don't break the top 25 in the App Store. 1,500 is probably very successful for WM but compared to Android and iPhone it isn't much.