Why?
ETA: iOS is arguably the biggest/most-popular gaming platform ever. The increase in sales from "I would have bought an Android/whatever but didn't because game X is only on iOS so I bought an iPhone instead" would be, relatively speaking, vanishingly small.
Ben Thompson does a better job explaining the why than I do: http://stratechery.com/2013/casual-gaming-is-a-sustainable-b... but the tl;dr is basically that the casual games most people look for on iOS are either already on or easily portable to Android or Windows. If Apple wants people to buy iOS devices to play games (and they should; that's a lot of people!), they need to make sure there are good games on iOS that people can't find anywhere else; otherwise, people will buy cheaper Android/Kindle/nook devices instead.