Firstly, a list of those reasons I might not have chosen GURPS:
- GURPS is new-school: Or at least it has new-school elements, whatever that means. It has skill systems rather than relying directly on player skill. It takes time to build a character, and it's expected that that character will be role-played at least a minimal amount. Contrast this with five-minute character generation and no necessity for role-play found in OD&D (at least to judge by reviews and retroclones) up through 1e. Additionally, a lot of the advice that comes with the core rulebooks is new-school, encouraging things like pallete-shifting and adventure design as opposed to objective situation design. (B502)
- GURPS is complicated: GURPS has a lot of bells and whistles not present in (most of) old-school D&D and retroclones. GURPS doesn't abstract combat; each sword swing is a sword swing, each combat turn is a second (that overlaps with other turns). There are hit locations and defenses and finicky difficulty modifiers etc.
- GURPS requires conversion: Most of the OSR stuff or original stuff picked up by the OSR can't be run out of the box with GURPS, like it can with the D&D family. Want to run B2 in AD&D or S&W? Sure, no problem. In GURPS? You're going to spend about as much time to convert things as it would take an experienced GM to write a like adventure himself.
- I'm comfortable with GURPS: I don't mean I know every little fiddly rule, or even that I'm totally comfortable with some of the core mechanics. (Is the minus to hit the leg -3 or -2? -2, but I always forget.) But generally, I have a sense of the system, of what modifiers make sense, of what character abilities are useful for what, etc.
- My players play GURPS: I might be able to convince them to pick up Swords and Wizardry for a change, if I really wanted them to, maybe. But then, we're in a group with three GMs, potentially more, so it's entirely possible they'd go, "Nah, let's play this other game that uses the GURPS rules instead." This and the first one are the two biggest reasons.
- GURPS is old-school: Yes, it's both. GURPS is put together like a tool-kit, and is very up-front about the fact that if you don't like something, you should drop it/change it/house rule it. Don't like the shock rules? Drop'em. Don't know the modifier for hitting your enemy in the vitals under a full moon at 20 yards? Eyeball it. Want DR values to be different? Change'em. Steve isn't going to come to your house with a shotgun, and the other GURPS books will still be useful for your game. Additionally, a lot of the advice that comes with the core rulebooks is old-school, encouraging improvisation and the mindset that the rules are tools, not a straitjacket. (B497, I'm looking at you.)
- GURPS is simple: Yes, it's both here, too. GURPS has a lot of fiddly bits, but the core mechanic is easy: roll 3d6, roll below a number. "But," you say, "that's not a real representation. By the same logic, D&D is, 'Roll 1d20, roll above a number,'" and you're right. But the overall system of arriving at the number to roll against is coherent across all uses, and is easy to internalize as an intuition. Sure, you won't be exactly correct making up an on-the-fly ruling compared to totalling up all the published modifiers, but I'll bet 90% of the time you'll be within +-2.
- GURPS has good support: GURPS is classless and designed as a huge tool-kit out of which you pick what you want, which could present a problem, but the Dungeon Fantasy line of PDFs does a wonderful job both of trimming out what you don't want or need for your dungeon crawl as well as supplying things that didn't exist before for GURPS but are quite useful for D&D-esque play. Dungeon Fantasy 2, Dungeon Fantasy 8, Dungeon Fantasy 15, Dungeon Fantasy Monsters 1, and others. In fact, the support is so good that I'd likely use about half of what's written for GURPS even if I were running a different system.
So bring on parrying and blocking and all-out-attacks and called shots to the head and threshold magics and negative HP totals. Just don't forget you need a mysterious, dangerous place and torches to see it by.