2006 was the last year GURPS was a top selling rpg. That’s two years before d&d 4e and 4 years before apocalypse world.
There was about a decade where 80% of SJ Games revenue was Munchkin.
GURPS died a long time ago because it never evolved. It’s exactly what’s it’s always been, and that still works for some people, but the industry has evolved and grown and there’s a lot more options and play styles now.