My recollection mostly had to do with the old way Qt was licensed, which affected how people wanted to include KDE in distros. Gnome managed to step into the void by leapfrogging other choices like CDE (way back!) and it managed to get wired into a few fast growing distros. Most notably, it was pulled into Ubuntu due to the Qt licensing on commercial distros, then many things based on Ubuntu, and here we are.
I’m sure there were other considerations about features, where Gnome had a good set of tools, but used to be lighter duty than KDE. There was also a window of time where Gnome was designed to be more touchscreen/tablet friendly while KDE stayed away from that style (good!).
Different licenses, different styles, different release times. A bit of “right place, right time, now the default” for Gnome.
I like KDE, but I’m mostly a Mint/Cinnamon user, and have been around since SunOS CDE systems, so it’s all better than that! I’ve got a couple of kids on Ubuntu/Gnome, mostly due to driver issues.