[JoshTriplett]

Concept association. Most books, academic papers, and other printed materials use serif fonts; a serif font thus tends to convey a more authoritative, dignified, academic look, more strongly associated with traditional publishing and all that implies. That's what you want in many contexts, but not if you want to give an impression of "so cutting-edge it isn't in print or other traditional media yet".

It isn't a rational reason, but it's a very real one. See also the studies on the perceived authority of various fonts (Computer Modern and similar near the top, Times and similar in the middle, Comic Sans at the bottom).

[tsironakos]

I couldn't have said it better :)