prosfilaes
Adventurer
Name recognition.
That's it. I guarantee that if nobody had heard of Dungeons & Dragons, and the name everyone had heard of was something else, we'd be having this discussion about a completely different game, and D&D would be in a position like Tunnels & Trolls or Rolemaster or something.
...
D&D's core rules systems aren't superior to other game systems. Most people could find, or be presented with, a system they would find they like better.
I don't think that's the end all and be all of it. Sure, I can see having this discussion about Tunnels & Trolls or Rolemaster or GURPS*. But setting and genre matter; offer something too weird like Empire of the Petal Throne or Gamma World, it could have been first, and lit off the RPG industry, but it would have been surpassed by something more normal. I don't know if it had to be fantasy; maybe it could have been Traveller or Hero, but I don't think the Cthulhu Mythos (at least not at the time) or steampunk or cyberpunk or Western or too-odd science fiction would have sustained the game. Fantasy that had a large element of Tolkien pastiche was possibly the best chance for the first RPG to have massive staying power.
* I know, G(eneric)U(niversal)... but the 3rd edition rule book could stand alone as a fantasy game, albeit human only. In an environment where GURPS was first, it probably wouldn't have spent as much time being weird, because more of its players would have been interested in standard fantasy, and its players wouldn't have been running from D&D.