Or more likely because Rust was developed to support Windows 7 before it was deprecated; Swift never targeted Windows 7. Swift was open sourced in 2015, the year when Windows 7's mainstream support ended; at that time, Swift only targeted Mac OS X and Ubuntu.
Rust began as a project in 2010, Windows 7 was released in 2009.
I think GP's comment is fair. The portability of Rust is of a completely different order of magnitude than Swift. I mean, people have actually gotten it running on Windows 98 SE. And on Windows, Rust has access to a rich winapi surface, nice COM wrappers (including semi-official support through Microsoft's com-rs), a WinRT projection. Not to mention that crates basically just work. Maybe it isn't a first-class supported language for Windows just yet, but it's starting to feel pretty close.
This Rust/WinRT translation is the key for Windows maturity now. C translation is the everywhere basic but Windows support is achieved with eh WinRT translation.
And right now, that is C++, JS, Rust and .NET. Not more not less.
Rust began as a project in 2010, Windows 7 was released in 2009.