
The reasons are mutually exclusive?
First, best known, open to new players. All coming together to produce an evergreen product. Much as I disagree with certain aspects, Wizards did right when they put the focus back on D&D's Core Story. It's all about the adventure.
BTW, I was trolling after a fashion. I wanted to see what sort of responses I'd get, and I was not disappointed. I will say WizarDru did give the best reason, D&D is fun. You play it to have a good time. Not every adventure, not every playing session is going to give you the happy fuzzies, but a good session will leave you with a positive experience. Even if it's a TPK.
A TPK?
A good TPK is a source of legend.
When you're dealing with why people do things there's rarely just one reason. We tend to complicate things for ourselves. We got into D&D because we heard about it, we found people ready to take us in, and it was fun. Had any of these been missing D&D would be known to a very few and Col. Pladoh would be an insurance adjuster. {
You know, that might not be such a bad thing. Emily, I heard the Parks Department is looking for you.
Eek! (sound of sauropod speedily tiptoeing away)}
Emily Bronte-saurus, the bane of my existence. Anyway ...
... amending my hypothesis I present the four reasons why D&D is more popular than any other RPG: Precedence, exposure, openness to new recruits, and fun. But the chief of these is fun.