GNUStep runs on Windows, and it's about 90% compatible with the Cocoa API. I think there is a demand for Apple stuff, not a demand for their unusual API.
As someone who's developed for Windows, Linux, and OS X, I think it's reasonably safe for me to say that this is true. There are a few people out there who genuinely do like the Cocoa API, but they are a very small minority, composed almost exclusively of Apple fanboys.
I should probably qualify that a little bit, though. Cocoa isn't terrible, and neither is Objective-C. I would much rather write GUI code with Cocoa/Obj-C than with C/C++. However, both Java (either SWT or Swing) and C# are (slightly) more pleasant to write code for. That said, you can't write iPhone apps in Java or C#, and neither Java GUI toolkit gives you a sufficiently native feel on OS X unless your app is extremely simple.
If given the option, I would happily switch to another language/toolkit if it gave me full equivalency to everything I've got in Cocoa, but without the pain of manual memory management and the arcane syntax of Obj-C.
(a) the guy with the idea behind this bill was "open government", "open access to court records", "open source", "open everything" activist Carl Malamud, who was most recently in the news when Congressmen and Senators started picking up his thread about making PACER -- i.e. court records -- free (as in beer); and
Also, it should probably be noted that Carl Malamud is informally campaigning to be nominated as Public Printer of the United States.
The closest to perfection a person ever comes is when he fills out a job application form. -- Stanley J. Randall