There is a Linux version actually (Flatpak, Steam, as well as often being packaged by individual distros).
There isn’t a Linux version on itch.io since it would require either for us to include all Wesnoth’s dependencies as part of the download (which is a pain) or require players to download all the required dependencies themselves (also bad). Flatpak and Steam on the other hand provide all the dependencies that Wesnoth needs as part of their respective runtime environments, which makes it much easier to provide just “a Linux version” there.