There is X and now wayland. If you are doing something beyond basic you can quickly fall into needing two different implementations for GUI things.

There is jack, pulse, alsa and now pipewire for sound. On the system you can have multiple sound devices some available fully working only through pipewire, others fully working only through pulse.

If you want your app to integrate into your desktop nicely (basic thing like using tray icons) you can have completely different behaviors on different distros because there is so much variability.

Pipewire is now taking over a lot of things regarding audio/video. With fedora already fully on it. Great but the documentation is in a miserable state. Very few examples. Some distros while have pipewire running, using it for audio will just not work.

Understandable. It would be nice if a lot of the Linux DE were consolidated, at least for a "target" for developers. I'm genuinely hoping SteamOS 3 helps with this. Of course, this consolidation will come with issues as well, as we've seen with the dominance of the Chromium engine for browsers...