Ok, that's simply not true. On Mac OS X, you're welcome to use any language and runtime you wish for your application. Hell, if you want to use a non-Objective-C language and still present a native experience, use PyObjC, RubyCocoa, MacRuby, or any other language that shims into the Objective-C runtime and get access to all of Apple's Cocoa frameworks in the language of your choice.
The same is possible on iOS as well, with the (significant) condition that Apple must first explicitly approve your use of a non-Objective-C runtime before your app can be posted to the App Store.
But in direct response to your post, Obj-C has a rather level playing field on OS X and still survives vs. the alternatives.
PyObjC isn't, but ships in Mac OS X. That's rather officially supported.
> * it binds to Cocoa (and if you're lucky it's even up to date and doesn't have that many bugs)
Not true. MacRuby is a prime example of a language that taps directly into the Objective-C runtime, eliminating the need for a bridge. And PyObjC is a great example of a mature bridge – it's been around and robust since the NeXT era.
Those projects , even macruby are not officially supported - there is not even documentation on apple.com . Who would use it as their main dev platform, knowing that apple has the tendency to ban 3rd party platforms. Obj-c is practically not used outside OSX - there is just no evidence that, when given the choice, developers would prefer objC. Open source developers seem to root for python, ruby etc, but certainly not for objc, unless they have to deal with OSX. GNUStep is not an active project, with practically zero applications, while there are lots of quality apps in c++ frameworks, python, ruby etc. I dont believe OSX developers are just different people who happen to love objc over everything else.
The same is possible on iOS as well, with the (significant) condition that Apple must first explicitly approve your use of a non-Objective-C runtime before your app can be posted to the App Store.
But in direct response to your post, Obj-C has a rather level playing field on OS X and still survives vs. the alternatives.