One of the things I learned during my GURPS game was just how brilliant the regular measured reward system used in D&D is.
D&D hands out some cookie, experience, a new level, a new spell, magic items, something at every sessions and this is a powerful incentive to continue the game for longer.
While GURPS can be played this way (c.f Dungeon Fantasy) its not the default play. You often get rewards based on what ought to be there rather than Gygaxian Naturalism. This lack of reward, smaller experience awards (1-5 matters but it feels paltry) , less magic items and less treasure takes away a big incentive to play longer.
This can lead to more player boredom , shorter campaigns and more a tendency to "move onto something else" as happened to me . This isn't bad necessarily, games can go on to long and it can even be a real plus especially in todays entertainment option rich world
Still if a longer old fashioned game is whats desired, well its going to require some accomodation
I think its also why GURPS, while an excellent system (been in the world for 26 years) has never reached the popularity of D&D. Its fun but its not the fun most players are looking for.Dungeon Fantasy
of course works to correct this and does a decent job of it, but its still GURPS and while its a loving tribute, its also in my opinion a bit snide too.
I guess its an issue of the Right Tool for the Right Job. Use D&D to play D&D, Use GURPS to play GURPS