Mostly because the system itself is completely forgettable and outdone by multiple other generic systems, not to mention systems created for the game. The only distinguishing characteristics are 1) its dozens of splatbooks of widely varying quality and 2) its extreme emphasis on character creation.
In the free Lite PDF, for example, pages 7-17 are solely on advantages and disadvantages your character might have, including being slightly stinky, illiterate, or not liking to lie. These disadvantages give you points that you can spend on being stronger or mightier or having one of the dozens of lovingly detailed skills.
The rest of the PDF, obviously, is on the details of armor, weapons, and specific fiddly rules for fighting (pgs 25-29). By contrast, it gives a shred of a paragraph on world creation.
So, that's the game.
That's what people are telling you to play everything from a dungeon crawler to a high-flying kung-fu game in; a game where everything should be written down and detailed as options for you to take. That's the game.
Never mind the fact that games like Mini Six can have you generate a character in half as many steps with half as much fuss and still leave you plenty of wiggle room to make up rules. Or that a game specifically for the genre will have more support for what you're trying to do through rules and advice and neat little subsystems that fit the tone of the game (D&D, Donjon, Kill Puppies for Satan) , or have a cool resolution mechanic (Dogs in the Vinyard, Donjon again, Savage Worlds, Fate, Unknown Armies, Dungeon World), or let you make up stuff for your game without worrying about balance for everything by being lighter and out of your way so you can have fun (like Mini Six, or Risus, or D&D again). No, it's important that you mechanically create a character that's point balanced in every way using a complex and oddly fiddly system so that you can play the complex and oddly fiddly combat subgame or maybe roll a reaction table.
"But Nick," you say to me, "you can just ignore the rules you don't like!" Well then why do I need the rules at all, smarty pants?
"But Nick," you ask me, "it sounds like you don't like generic systems at all! There's nothing wrong with GURPS that isn't wrong with other generic systems!"
Yes, there is. It's the fiddliest, which means hardest to customize to my liking. If I wanted to run GURPS Dungeon without actually buying GURPS Dungeon, I'd have to write a book the size of GURPS Dungeon with half a dozen skills and advantages and disadvantages and by that point, I AM REWRITING THE GAME. What is the point of rewriting the game every time I want to do something?
And no, you don't do that in every generic game. In Mini Six, for example, you can offer a handful of skills and everybody picks some and then you're off. I wrote a scenario for Mini Six that made a modular weapon system and was in a sci-fi world and the rules changes took all of two paragraphs. Other than that, I was running standard M6, because that's how the system works. It's light enough to get out of your way.
In Risus, well, there isn't anything to rewrite because you can make things up on your character as you go along and the game supports that. You can make stuff up in a way that you can't in GURPS, because the system is designed around that. You want a truly universal game, you want Risus, because it doesn't take ten new books to play a cyborg half-rabbit jedi-slayer riding a robot killosaur.
That's why GURPS is so frustrating. It's objectively the worst generic roleplaying system but it's the only one people ever want to talk about. In a world where Action! or Active Exploits or Donjon or M6 or any of these other systems do generic better than GURPs, why is it the only one to get any love?
"But Nick, I like GURPS! And my group likes GURPS! I think you're just saying all this because you've never tried it!" You're right about that. I have never tried it. And I'd be more than happy to play in a game where somebody was running GURPS, because I'm not a dick to people who want to play games with me.
But if somebody asks me my opinion, look out.
Short List of Better Games Than GURPS
Before somebody calls me out, here's a short list of games that do things better than GURPS. These games are all free or cheap.
Risus: The lightest rules you can still use
Swords and Wizardry (Whitebox): Light, easily extendable rules that, like a good bikini, cover just enough
Donjon: Generic rule set that lets you do what you want with no fuss or muss
Dungeon World: Clever, cinematic rules, surprisingly light, extremely engaging
Active Exploits: Cool diceless gaming, go look at it
Dogs in the Vineyard: Poker style resolution really amps up the suspense, great setting details
Legend of Wulin: High flying wire-fu is actually supported by the system
7th Sea: Unique setting, ORE engine is simple and awesome
Reign: See 7th Sea
Warhammer Fantasy Battles: Dangerous magic, fun setting, good character gen, deadly combat, interesting arms and armor system
Mouse Guard: Team-based gaming in a unique setting, new and fresh resolution, cool character gen that ties you into the world
Lacuna: The coolest roleplaying book ever written. You can't not want to play this.
InSpectres: See Lacuna. Same deal. This game is awesome.
Warrior, Rogue & Mage: Your character is exactly as detailed as it needs to be to go kick ass
So there you go. If you want to see a couple of games that I think are better than GURPS (and that deserve more attention than it gets), there's the short list. I'm sure I'm missing something great; feel free to let me know about it.