I’m a huge Swift advocate, even on the server, but I have to admit Swift for Linux feels like an afterthought sometimes. For instance there are still no official Debian-based Swift docker images, and the “stdlib” (the Foundation module) is very often behind the (private) macOS’ one (still no support for async/await, lots of bugs that take time to be resolved, etc.)

Things do get better with time and developing for the server with Swift is a great experience, but it still feels a bit early.

The complete lack of any explicit support from linux vendors and cloud providers.

For more popular languages you get predefined templates for serverless platforms and compiled binaries directly in the linux distributions default repositories.

In many ways the outside interest in swift died down even quicker then the interest in running ObjectiveC outside of the Apple Ecosystem did, and as the Apple ecosystem don't include an server platform all swift server-side code is running outside of the apple ecosystem.

The problem likely isn't that swift is bad just that it's unremarkable once you take it outside of an iOS context.

Adding on to the other comments, swift on Linux also does not have some frameworks like Combine