My theory on this is the following:
Dinosaurs existed when the earth consisted of a different atmospheric mix; certainly most went extinct because of tripping and falling in a tar pit but a lot gradually lost the ability to breathe.
As a behavior adaptation, some of the reptiles moved back into caves since their exhalation consisted of gases that, when hitting the increased oxygen, set stuff on fire. Sure, sounds like a great defense mechanism but awkward when you prefer your small furry critters medium rare.
So St. George killing the dragon… the dragon totally let him.