aramis erak
Legend
That's exactly it. It's about branding. There's nothing unique about D&D; but it does its job well and is more well known as a term than "tabletop roleplaying game" is. Hell, I know people who say they're playing D&D when they're playing something else entirely.
the brand is also much bigger than the RPG. The novels supposedly make at least as much as the RPG line. The videogames more than either alone.
The related tabletop games also do a nice chunk of change.
D&D isn't JUST the RPG; it is a whole collection of cross reinforcing brands and games in all aspects of gaming.