Maybe rather than being about difficulty, it has to do with patents? No idea, just a hunch.

You could start by defining "linux" app.

Linux is not really a thing. It's a mess of numerous operating systems. Ubuntu, Debian, CentOS, redhat, slackware, all themselves available in different incompatible editions.

I pity developers who have to support desktop applications on linux.

Not exactly. The only things you need to handle are :

- An old enough glibc (glibc is backward compatible)

- static link as much as you can (I know this is against the unix way, but the only way to workaround most of the compatiblity issues). This is pretty much what macOS apps do by shipping .framework within .app.

Beside this, QT and GTK will pretty much always be available.

Way back in mid-2k (maybe late), Skype was about to release the core of the code for linux as a lib that could be linked to from 3rd-party chat apps. They nixed that at the last minute (after getting a lot of partner's hopes up) and instead we ended up with a shitty app for Linux instead an ok lib for Linux. I only heard about the lib as I was working for a company that was running the Skype hardware stores at the time.

They only need to do a part of the work. The other part is the repo maintainors job. These many distros also boil down to a few fundamental distros.