I think it's because they have a great UI, they respond to feedback, they've worked hard at marketing it, they impressed some VCs, and all the million other reasons that start-ups succeed. Don't complain about it, ask why.
I'm sure the team is great and the investors are great, but let's be honest, the product is fairly light right now. The UI is nifty, but there's nothing special about it.
I'm not trying to knock the guys, I'm simply trying to determine why they've become such media darlings without really having delivered anything.
http://teamtreehouse.com http://tryruby.org which I believe is an extension of http://codeschool.com http://learncodethehardway.com
Then of course there are sites like Peepcode, Udemy, and a host of others that offer tutorials, which aren't quite the same, but stil serve the same purpose.
I'm sure there are more I don't know of.
We can build causal histories but maybe they exist just because they make sense not because they're true.
EDIT: I added the second paragraph.