In the words of Ryan Dancey (who used to be one of the bosses of D&D in the early 00s): network externalities. Basically, D&D is big to the point where for many people it's synonymous with RPGs, so if you're looking for players you're likely to find some. That turns into a big positive feedback loop. If you're looking for GURPS players, you're going to have to look far and wide – in my experience, it's uncommon for a group to start off with a non-D&D game. It's more likely that an established group will go "Eh, this RPG thing is fun but I'm getting a bit tired of D&D. What else is out there?". And of course, it's quite likely that those groups will wander off in different directions. Some will try Call of Cthulhu for a real change of pace. Some want a game that's sort of the same but crunchier and go for Pathfinder. Others like the sci-fi/space thing and try out Star Wars or Starfinder. So basically, the small portion of the gaming market that isn't D&D gets split up dozens or hundreds of different ways.