Don't get me wrong, I like GURPS, and I do think that realisticly the market it targets will never be as big as D&D for the simple reason that the mechanics are much more complicated. And, I think that is the principal reason GURPS has always been a top 5 game system but never the darling of the RPG world.
But, I have always had the suspicion that especially in the early going alot of the reason GURPS failed to become quite the phenomenom of the TSR games is that GURPS failed miserably in offering packaged adventures. Compare GURPS to Star Wars, Call of Cthullu, Chill, Shadow Run, White Wolf, D&D, Gamma World, Paranoia and many other games that offered very strong very well written modules and you'll notice a marked lack of quality in the adventure writing. A module has the advantage of providing direction to a starting DM on how to write an adventure - what elements to include, what kind of problems to solve, how to describe NPC's succintly, and even how to organize it. I have always had this suspicion that most GURPS books that get sold go onto DM's shelves, get read by DMs, get flipped through by DM's, get pondered over and fantasized about by DM's, but that for the most part they remain a sort of DM's toy that they tinker with from time to time but rarely actually organize a long running campaign around. I confess that I've found it awfully hard to write a GURPS adventure, have never successfully had a long running campaign in the GURPS system, and find the principal use of the material is stealing ideas for use in other game systems.
I wonder how many GURPS books exist per actual group of players that meet weekly to play the game?