I went walking along a local beach the other day and discovered some caves. Naturally, I took a look inside; some of them stretched back a good 20 yards or so.
Despite the fact that it was reasonably good weather and the beach was busy with dog walkers and sea kayakers, and it was the middle of the day, it was nonetheless easy to be reminded that human beings are not made for life in dark places. More or less as soon as you enter a cave like this, you are out of your comfort zone: you can't see, it smells wrong, you feel disorientated and directionless, and there is a vague sense of irrational fear that something may be lurking in the hidden depths.
I think part of D&D's success can be explained by the fact that there is something visceral about the concept of being underground, in the dark, where there are monsters. We can all envisage how strange and disconcerting it is not to be able to see and not to be able to sense, in little subconscious ways, where you are and which direction you are going - through things like the direction of the wind, the position of the sun, or distant sounds. Instead you feel entombed, shut in, and your mind can't help but speculate that there is danger lurking somewhere near. Everybody understands that kind of experience: we know what it is like to be afraid and alone somewhere dark.
Would RPGs have been successful if the first RPG had been Traveller, or Vampire: the Masquerade. Possibly. But I think one of the reasons for D&D's intrinsic appeal is the fact that it is about dungeons: there is something primal there people get when they hear about it.