It was revealed some time ago in a documentary series cartoon on Nickelodeon.
To wit:
[WC = Wilbur Cobb]
WC: I know what really killed the dinosaurs: TV! Too much brain-dead TV! Turned them into a a lot of, you know, a bunch of slack jawed zombies, then they all drank out of a dirty dog dish, a disgusting habit. Oh no, no sorry its because they all moved to Jersey, yeah thats it. Haircuts, yeah they all had bad haircuts...
Ren: I still say think this is a load of crap... thats all he gives us, crap, crap crap, man I'm bored.
WC: Then they went and ate clams in a month without an 'R' - without an R yet! As a Union man, local 842, I find that shocking! Utterly shocking I cant believe it!
(Stimpy takes WC's hat, eyes ears and nose and put them on)
Stimpy: Look Ren, blah blah blah blah blah, this and that, blah blah blah blah blah
WC: Then they all went swimming, minutes after they finished eating... No! they thought they were superman and jumped off a building. Ran around with scissors in their hand, then poked their eyes out with grapefruit spoons... Athletes foot- NO! Hemorrhoids! Really bad hemorrhoids! Oh oh I know, they sneezed, belched and coughed at the same time! And they all exploded!
Stimpy: Wha!
Ren: It will all be over soon pal...
WC: Well it was a Peruvian Butt Plug... yeah and they all had to eat each other to survive! he he thats what happened... Super heated gases shrunk their lungs to the size of raisins! And the kid on the top bunk died!
Security guard: Hold it! Don't listen to this guy kids, he ain't nothing but a lowly bone polisher around here. Back to polishing, Cobb.
WC: No no, it was me! I killed the dinosaurs! Me, I did it! Mwahaha, with my bare hands, the mighty Cobbersaurus! I ran them over with my truck!