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, according to Mel Plotsky, sales manager for the David Berg hot dog company in Chicago. (Chicago is one of the hot dog's holy cities.) 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.
For that matter, you want to watch the mustard, too. Plotsky says your mainstream brands like French's put in too much turmeric and whatnot. What you want is some unpretentious mustard like Plochman's that enhances rather than competes with the flavor of the beef. You should also steam or grill rather than boil your hot dogs--water leaches away the flavor and softens the wiener till it becomes non-tooth-resistant mush.
But--getting back to the original question--you say you like the taste of tomatoes. Fine, then eat tomatoes, as God meant them to be eaten--fresh sliced and piled on top of the hot dog. The recommended ingredients of a hot dog with everything, in order of application, are mustard, relish, chopped onion, sliced tomato, kosher pickle spear, optional peppers, and celery salt. (Many think you have to get kraut in there too, but Cecil wants a hot dog, not Oktoberfest.)
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."
But hey, if you want ketchup, by all means get it.
