It's because the tool has been made in Windows Presentation Foundation, which is a .NET UI framework targeting Windows specifically. It's been good enough for now since the jams it's used for require a Windows or web build anyway.
I plan to port it over to Avalonia, which is more cross-platform, but it will take quite a lot of effort, testing and tinkering before then. Since it's currently a one-person side project, I haven't been able to make the shift yet.