Short Answer: Marketing departments and monster movie fans.
|Above: Not what most people think of when you say "dinosaur."|
(Ashdown Maniraptoran by Matt Martyniuk, all rights reserved).
This is a philosophical issue that's been on my mind for a while now, inspired by some recent and heated debates over the content of the Dinosaur article at Wikipedia. It also seems to be simmering in the background of a lot of discussions about the recent suggestion that Jurassic Park 4 will not feature modern, scientifically accurate dinosaurians.