When I first started playing RPGs, i wasn't playing RPGs, I was playing D&D. (And it really was the first RPG I played).
Even when I was playing other RPGs (Traveller, CoC), to non-gamers I was playing D&D (because people had heard of it),
Attributing D&D's market share solely to the nature of the system is to ignore the advantage that comes from being the first mass-appeal tabletop RPG (in a pre-internet era, when non-geeks didn't play computer RPGs because they [Computer RPGs, not non-geeks
] didn't exist then. Yes, I am a geek): Great Aunt Annie walks into a games shop to buy something for her nephew, gasps in surprise at the range on offer, but then buys D&D, because that's the one she's heard of. Maybe.D&D also offer a generic / specific background - it can be any sword and sorcery setting: a very specific world (and there are published campaign worlds for it, or one that the players have read of or seen in a film) or a generic world.
Other long running RPGs seem to use systems that are more or less complex, but benefit from a solid and playable game world:
CoC (the Mythos background which can be applied at almost any time in history - I finished running a CoC Rome mini campaign a bit over a year ago);
RuneQuest, particularly with the Glorantha background
Traveller (which I gather is not / was not that brilliant in recent iterations), with it's well developed imperium background
There's possibly a case for Vampire here too.(The world you know, but from a much different perspective)
But the other thing I take from the above is that they are all amenable to long campaigns. Yes, they can be used to play one-off scenarios or short mini-campaigns, but they can just keep going, if the players want to keep going.(And how long does it take to get a D&D character from 1st level to 20th?)
They also tend to have moderately detailed character generation, where how the character came to be is defined by the generation system or by encouraging the player to fill in the details. This seems to go against the idea of 'quick character generation and then jump right into the game' approach.
The 'quick' approach would seem to give you the 'physical' details of your character (i.e. statistics skills, etc), but the actual 'character' of your character only emerges as you play.
The more detailed approach seems to me to encourage a player to think about their character before they start playing, and thus have some idea of where they come from, their values and goals in life (Putting the 'Role' into Role-playing_)
Even in T2K, this can apply: the first goal in life is to stay alive, obviously, but in the last campaign I ran (a long time ago), after the characters were generated and equipped, we took a few minutes just to establish how come this particular group had formed as a 'breakout from Kalisz' group, which without forcing it established the beginning of a 'character' to each of the player characters, which then developed further as the campaign progressed. As someone almost said, they were not just Grunts.
Trying to make some of the above relevant for T2k:4e:
The T2k campaigns that I ran all petered out due to the Real World getting in the way, but the campaigns weren't short mini-campaigns such that had the PCs got to Bremerhaven I would have said, "right, you all sail away to the west, campaign over". My intention was always to run longer campaigns - step one, survive and get back to where the characters want to be, Step 2: rebuild. (The weakness in the earlier editions to me was the absence of anything much to help develop games that featured the 'rebuild' bit).
Now, I know that the Real World will get in the way and I'm never actually going to be able to run this campaign, but I would like to be able to run a campaign game where the characters that are celebrating restoring an operational water treatment works and wind turbine power supply to the town they are in include one or two characters that can regale their fellows with tails of clearing anti-tank mines near Krakow, and riverine combat on the Vistula.
