...and the webcomic, maybe more importantly, is self-published. Randall Munroe didn't have to pitch his comic's marketability to a syndicate in order to find an audience and make you aware of it's existence. All he had to do was produce it and put it out there, and it got popular because people liked it. It got so popular, that you can make jokes about it, and be reasonably sure that any given nerd picked at random will probably get the reference.

Can you conceive of such a thing happening even fifteen years ago? Back then, the only things to achieve such cultural relevance were centrally-distributed things like the Simpsons. Which had to be pitched to a network.