Because the officially supported platform, macOS/iOS, isn't a server platform anymore.

Previous investment in Windows architecture for one. Cross operability with locally running Windows applications for another. Unfortunately though there are obvious solutions to these problems, those who make decisions about spending are often not as concerned about doing it right (IE they already invested in Windows). It's a selling point for adoption. I suspected that with Linux Shell in future Windows Servers Kitura will just run there, but I'm sure there will be performance hiccups doing that (at least compared to native environments).

Obviously because you don't want to have servers running OS X?

Swift on Server runs on a Linux.

I know that. The parent asked why it matters that it's multi-platform.

Hence my answer: that it matters because if it wasn't, then you'd have to run Mac OS X to use Swift on the server side.

Developing on Mac, deploying on Linux, which could be x86 or ARM, etc? MacOS isn't really a server platform, definitely not a cloud OS.

Case in point, Apple's iCloud runs on basically anything but Apple systems (most notably, IMO, Microsoft Azure).

It matters that the same development tooling is available on Linux.