Economics, basically. It was costing me about $1.50 to market per sale, and for $0.99, just didn’t make sense to continue with it any more. For those that already have a copy, I hope you enjoyed it while it lasted. It should continue to be available for you to re-download, but no new purchases for iOS from this time onwards.
The game was originally developed on PC anyway, my original intent was to release on PC, but a partnership at the time moved things towards the iOS version. As that relationship has moved on, plus the technical and financial issues related to releasing on iOS, compromised the visuals and the game more than I liked.