compiling it for .NET on Linux

It always sounds so simple for the layperson. Just looking through the list of DLLs in the portable ZIP, PDN uses Windows Forms (System.Windows.Forms namespace), which, as far as I understand, isn't included in the cross-platform releases of .NET. They'd have to rebuild the entire UI with .NET MAUI or some other cross-platform toolkit. Then there are lots of other bundled libraries that may or may not work in a .NET-on-Linux environment.

Releasing and then supporting a second platform is never as easy as just compiling the executable. Figuring out the quirks and differences of various systems, especially one as heterogenous as the Linux ecosystem, is a drain on resources that small development studios often can't afford. Raiguard (hallowed be his name), one of the developers of Factorio, and the person who is single-handedly maintaining the Linux-native build, wrote a blog post about its challenges.

right, wasn't Mono supposed to handle things like this?

It makes no sense, .NET is fully supported on Linux and has been for years. There is an official SDK and it's capable of creating full standalone executables, or smaller ones requiring the runtime libraries to be installed on the target system. As far as I know good packages exist for just about any distro. .NET is fully open source and not just worked on by Microsoft, but a large community.

Creating a Linux executable from any .NET code is as simple as running a single command, it supports cross platform compilation as well. With support for a range of architectures and OSes.

The only caveat is all the code needs to be properly managed code, any unmanaged code is most likely not cross platform compatible and is thus not recommended. It can even break as drivers or the OS is updated for example, so it's a bad idea to have unmanaged code and usually not needed.