I've probably blogged about this in the past, but the question came up again and I've been giving it some thought.
Psion questioned, on TheRPGSite, the wisdom of using d20 for a planetary romance game, after finding that his Mars d20 game didn't exactly live up to his expectations. He's been running the genre with Spirit of the Century more recently, and been happier. At the same time, I was convinced to start another Pbp in a slightly unusual D&D-esque mileu. I'm using 3.5 with a few custom parameters. In doing this, though, I grabbed a concept that Rel came up with and is going to be running for his regular group using D&D 4e. So, the question is, why?
The answer comes in several parts, and one of those is quite simple: I'm lazy. I just don't have any interest in playing around with new systems anymore. When I was younger, system experimentation was fun, and I did it on purpose. Now, I'm more than happy to stick with a system that I know well, even if it doesn't do everything I'd like perfectly. Don't underestimate the benefit of system familiarity either; it's nice that I know d20 well enough that I can run it on the fly without much (if any) prior preparation to make sure I know what I'm doing. I also know it well enough that I can run it fairly fast and loose; utilizing the skill system to enable almost anything that you could think of as a player to attempt. Big benefit there.
Another benefit is the modularity of d20. Because of the open gaming license, tons of top notch game designers have tried their hand at d20, making up alternate rules that you can use for all kinds of reasons. Wizards of the Coast themselves put out a book of "official" house rule options called Unearthed Arcana that has a lot of cool options in it. The end result of this is that many of the things that I don't think d20 does well have already been more than adequately patched by someone... in many cases, I have more than one suitable patch to choose from, even. I'm not a huge fan of levels and the massive power discrepancy between low and high level, but if you run a game in a more limited setting, that problem takes care of itself (I rarely start at 1st level, and rarely run higher than 7th or 8th level before wrapping a game up.) And the E6 houserule set "officially" eliminates my level problem. I don't like how there's no AC progression to match To Hit progression, but UA has a houserule to take care of that. Various Action Points, Hero Points, Fate Points, etc. take care of many of my other problems with the system. Incantations and other alternate magic systems give me a magic system I like when the D&D default doesn't please.
A related benefit is the huge library of compatible material. This benefit is very hard to turn away from; I have access to literally thousands of feats, spells, classes, monsters, and everything else to choose from to use. In fact, this reason alone would be sufficient to keep me turning to d20 first, but the others ensure that I really have little interest in any other system most days. d20 may not be the literal "Holy Grail" system that does everything for me exactly the way I want it to, but I've had a really difficult time finding any other system that comes closer, and the other benefits of d20 give it the lead every single time I turn to run.