Linux desktop has a tiny market share. No reason to invest significant resources for that. Also, by supporting MAUI on Linux, Microsoft would feed their own competition in desktop area, and it's crucial for them to maintain dominance in that area. I think this is why they did not port the great WPF to Linux as well.

And besides, Linux desktop is a mess. I never learned details on how Wayland / X11 / KDE / Gnome etc. work, which is for what. All I know is that some distros switch from one to another, that one is declared the best, and another the worst, and all of it changes regularly. I'm glad someone in Canonical made the desktop mostly working for me in Ubuntu, although it still lacks a lot compared to Windows.

I think when you need that level of cross platform support to cover Windows, Linux, macOS, maybe mobiles, and you do a serious project, you use Qt.