There are many programming languages that in practice tend to be locked to one platform or another. C# isn't only on Windows and Objective-C isn't only on iOS, but very few people learn either of those languages with the expectation of "write once, run anywhere." (I've found that generally people who use C# on other platforms do so because they already know C# and don't want to learn something else.)

On principle I'd like to see Apple put Swift under a free software license of some kind and ideally make a cross-platform reference implementation, but realistically, even if they do that it's unlikely to ever get wide use anywhere but Apple products.