[Mithaldu]

At a guess:

Many iOS developers have a severe case of blinders on and don't communicate on their websites that their software is iOS only, ever. This does not only affect games, but also productivity software. The end result of that is that in the best case, a person gets to really like a product after having spent time on the marketing copy and when trying to figure out how to buy it for their platform find that they can't and end up despising the company. In the worst case the company uses some intermediary to sell the software, like humblebundle, which means the person can end up paying for the software, only to be told that they can only get an itunes version.

I don't know if that's the reason they do it, but in either case i applaud them for it.

[pchristensen]

It's also a sign to fans of the Apple aesthetic that it's more "Apple-y" than cross-platform products. See Tapbots vs Evernote for an example. For people that value that aesthetic, "iOS only" is a positive signal.