Paul, I know you don't mean to act like an alfalfa-chewing barbarian, but this is like asking why Leonardo didn't paint the Mona Lisa on black velvet. Ketchup is destructive of all that is right and just about a properly assembled hot dog (and we're talking about a pure beef hot dog, not one of those things you could serve with dressing on Thanksgiving).
Ketchup smothers the flavor of the hot dog because ketchup makers add sugar to their products. That takes the edge off the highly acidic tomatoes, but it takes the edge off everything else, too. Which is exactly why a lot of parents like it, ... Put ketchup on it and a kid will swallow anything — and from there it's a straight shot to Velveeta cheese, Franco-American spaghetti, and Deborah Norville.
...
People get pretty emotional over the ketchup question. Mel Plotsky opened our discussion by describing the condiment as a "catchall of garbage." Over at crosstown rival Vienna Sausage, they refer to ketchup as the "K-word." If you go into an authentic hot dog joint and ask for ketchup on your hot dog, the counterman will pause and look you in the eye. He may or may not say, "Ketchup?" with a tone of disbelief. But you may be certain what he's thinking: "Behold this creature that walks like a man. It wants ketchup on its hot dog.
http://www.straightdope.com/columns/read/679/why-is-there-no-ketchup-on-a-properly-made-hot-dog
BTW, we should have bought the book while we could:
It's selling for $114-$300!