Probably because it got popular as the easy Linux distro back in the 2000s and that label is sticking.

I remember that I attempted to install Debian on my laptop in 2009. It was ugly. I installed Ubuntu 8.04 and it was a totally different and much nicer experience. Because of that I've been on Ubuntu until they started pushing snaps very aggressively. I live booted Debian 11 and realized that its UI was exactly the same. I don't know when it happened during that dozen of years but there wasn't anymore a reason to stick to Ubuntu. I installed Debian 11 and got a faster machine with less background processes. I'm on Debian 13 now. I've been told that KDE is much better than what I attempted to use in 2014 so maybe I could give it a try, but it's unclear to me what I have to gain.

I prefer KDE (on Ubuntu, because I tried it and it's good enough) - it's got more stuff built into the OS in terms of settings. I tended to find that Gnome needs you to install more things to expose configuration settings, whereas KDE's configuration UI is pretty good.