The short explaination culled from a mailing list I belonged to ...
"As for voting for three different games I'd like the group to play, I'd nominate d20 Rokugon, Forgetten Realms and Scarred lands."
Funny how this player showed up to the game with the entire FR 3.0 library in his car.
Or for my medium explantion boiled down real life comments I have said and heard .....
Von:
"Hey do you want to play 'XX' its got a cool setting and ..."
Players:
"Nah, we'd rather play D&D"
Von:
"Hey do you want to play 'd20:XX' its got a ..."
Player"
"Sure, can I bring about three friends along?"
I have noticed pretty much that if a decent GM offers a d20 game with a chance to kill stuff, the players will pretty much take any setting offered to them as window dressing and form a waiting line.
Now for my essay.
The biggest things going for D&D D20? 1) Name recognition 2) Availablitity ...
I can only speak for myself. d20 have a very long history dating back to the granddaddy of all RPG, Dungeons & Dragons (aka Advanced Dungeons & Dragons).
It is a flexible toolbox of options.
I think these are close, but IMHO, the real reasons have to deal with three things.
1.) Money.
It seems that a lot of customers think the d20 trademark is a promise that everything under the book cover can be used in their games with little to no adjustment, so a d20 purchase is much less an investment of time and money compared to another systems. In truth, that isn't what d20 is about (its really about WotC saving its money by not getting involved in tiny projects it can't justify at it's level of business while selling more core books). But just as GOO about how many customers complained about how they were "betrayed" that they couldn't just drop the JLA into their Forgotten Realms game with SAS.
2.) Convenience.
Perhaps the best thing d20 did was to be sold as D&D.

Trust me, in old D&D it took pages to fit a wierd setting into the game (Dark Sun, etc.) Now it's a lot easier becuase of the tool box approach. WotC managed to bring the majority of the hardliners and lost customers back to the fold by selling a more up to date game to everyone. The lost customers saw a lot of the 1990s rpg mechanics they liked in other games in one place, and the hardliners got to pretend they were only "adjusting" instead of learning a new game.

Another part of the comfort zone is that D&D's best selling settings are pretty much the same setting, cosmopolitan pseudo Dark Ages. You just grab D&D, make a character and you go. No need to really learn a new setting (oh, okay these guys in this world are the Czarist necromancers, while in last week's world it was Roman necromancers). Helps a lot for pick up games.
3.) Storytelling?? In my personal experience, most PLAYERS couldn't give a spit about story, that's just the excuse to get to the next fight. Any game rules can be made to tell a story, and decent rules can be made into a combat exercise, just as any Sabbat and Werewolf player. I am not surprised that more players don't just openly giggle when a GM talks about story.