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.
The Wikipedia article on dinosaurs was overhauled in recent years to include the consensus scientific opinion that birds evolved from a group of theropod dinosaurs and should therefore be classified as a subset of theropod dinosaurs themselves. Despite the protests of a few diehard Linnaean taxonomy fans, this has pretty much been implemented. The challenge was then to write an article that essentially serves two purposes--address the topic most people expect to see when they search for "dinosaur" in an encyclopedia (big, dead lizards) while including the basics of the entire field of ornithology. Essentially, the article needed to provide facts and discussions that could apply to all dinosaurs, birds included.
The result, as you can currently read it, is a bit of a nightmare shot through with hedged statements like "most dinosaurs" this and "many dinosaurs that" and "some extinct dinosaur groups are notable for" another thing. The entire article could very reasonably be edited down to the following: "Dinosauria is an extremely diverse group of vertebrates which includes pretty much any anatomical feature, diet, lifestyle, or mode of locomotion you can imagine. For more information please click the links to the various, better-demarcated sub-groups."
I'm starting to believe that Dinosauria, as an extremely popular taxon to talk about, is actually pretty useless. My experience at Wikipedia has suggested that there is nothing really useful or interesting to say about the group as a whole, outside arcane anatomical details.
In practical experience, is anybody really interested in "dinosaurs"? I always say so, but that's more because it's a term people are familiar with. I'm far more interested in pterosaurs than I am hadrosaurs, but pterosaurs are not dinosaurs because Owen chose not to include Pterodactylus in his definition for whatever reason. I am interested in the overall diversity of animals along the line leading to modern birds, but admittedly somewhat less interested in modern bird themselves. So it would probably be more accurate to say I'm interested in stem birds (or, more broadly, pan-avians avemetatarsalians, ornithosuchians, dracones, or whatever you want to call the entirety of bird line archosaurs) than it is to say I'm interested in dinosaurs. I'm just as interested in dinosaurs as I am in eusaurischians or dinosauromorphans.
There are scientists who specialize in studying dinosaurs, but in reality they almost always specialize in one of the various subgroups that actually share close morphological similarity. Mike Taylor is not really a dinosaur paleontologist, he's a sauropod paleontologist. He studies sauropods. Sauropods are a fascinating and unique group of vertebrates that evolved from members of the clade Dinosauria, so we include them as dinosaurs even though Owen did not (just like birds). But when we specify that yes, sauropods are dinosaurs, well, who cares?
Non-specialists. "Dinosaurs" are a cultural artifact as much as a scientific term. I worry that if I tell most people I am interested in stem birds, they would have no idea what I'm talking about. So I say I'm interested in dinosaurs, and in particular the evolution of birds and flight. "So, pterodactyls right?" Now I'm obligated to say "No, pterodactyls are not technically considered to be dinosaurs for arcane reasons I could never hope to explain in casual conversation." Even though, yes, I'm also very interested in pterosaurs. This is a terrible state of affairs for science communication.
Who else cares about dinosaurs? Fans of Jurassic Park and other monster movie franchises. At some point during the 20th Century, dinosaurs became a cultural institution. People became fascinated by these giant, lumbering, reptilian nightmares that once roamed the ancient Earth.
Except they didn't. The animals most people think of when you talk about "dinosaurs" never existed. They're as "fictional" as Brontosaurus, if not more so. Brontosaurus is a real animal that had it's name changed. Dinosaurs are a kind of animal we once thought existed, but our ideas about their appearance and behavior have changed so much over the years that they're no longer recognizable to the general public, to the point that dinosaur movies like JP4 must go out of their way to stick to old, "classic" versions of them to satisfy demand for big, reptilian monster movies. We just don't have the benefit of changing the name, of leaving the label "dinosaur" attached to the outdated relics of the past it is synonymous with. People love big, reptilian monsters. They probably always will. The fact that they were once thought to be real added mystique. Now we're telling them those big reptilian monsters were really big weird-looking birds. "Ok," they reply, "but that's a completely different thing." People still want dinosaurs, and when they say dinosaur they mean Godzilla. Maybe someday people will think stem birds are as "awesomebro!" as dinosaurs, but that day is a long way off.
I'm not suggesting we get rid of the term dinosaur or taxon Dinosauria. It's useful in some scientific contexts. But I'm suggesting that maybe we're better off ignoring it in contexts where a broader term or more specific term is more appropriate, especially in science outreach and communication. I can talk about my interest in stem birds and patiently explain how they include various things we know as dinosaurs as well as everything more like a bird than like a crocodile. That's fairly simple. I can be more specific around people who are already in the know.
Of course, I'm still happy to use the term in the title of my (shameless plug) Field Guide to Mesozoic Birds and Other Winged Dinosaurs. Let's not kid ourselves, "dinosaurs" move books!