Quite simply because the iPhone is the best selling phone in the market
Sure android as an os is more popular that iOS
But that market share is split up among what like 200 phones?
So which ones do they make it compatible with?
On the iOS front you have the 3GS shape, and the 4/4s shape
That's it
And that other advantage is the software upgrades, something alien to most android owners
Elementary my dear Watson