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Probably in that it's the language with first-class support on that platform. Adoption of the language is encouraged more by the target platform's support for it, than by its independent merits as a programming language.

I'm not sure I'd agree though; Swift shares this position with its predecessor Objective-C. Swift's success versus Objective-C can be attributed to its relative merits as a programming language.

Right, but it's much easier to be a successful language when you're only competing with one other language as opposed the whole field of options.

You have no other choice to access the platforms latest and greatest features and the ones you can (through Objective-C) are less ergonomic to use.

platform gatekeeping, at a guess. though the existence of objective-c makes that status for swift be not-quite-technically-true.