Because the officially supported language for iOS development is Swift (and ObjC) and Xcode is quite good for iOS development.
I also like Swift a lot. It's a modern language with lots of powerful features and also very practical.
My current project involves a lot of context switching between Swift, C++ and Objective-C++ and I miss a lot of Swift's features in C++.. For some obscure reason I also enjoy Objective-C++ quite a bit, it's a weird mixture of insanity but it feels good ;).
I don't know a lot about C#, I think it must be a good language, albeit older with more historical baggage, but even so I wouldn't use it instead of Swift for iOS development.
I mean, why ?
For Android development - maybe - I'm not too fond of Java, but on iOS/Mac - I will stick with Swift thank you.
