Because she's a tard. Mustard goes on a hot dog, not ketchup.
Better yet:
New York style-- all-beef Sabrett, Nathan's, Hebrew National or Boar's Head with brown mustard and sauerkraut
Midwest style (Coney)-- Coney sauce (thin meat chili), chopped onions, mustard, and (sometimes) shredded cheese. Texas wiener is similar.
Boston style-- Kayem frank with yellow mustard on a butter-fried crustless bun.
West Virginia style-- Topped with beef chili and coleslaw
or my personal favorite, Chicago-style-- Vienna beef frank in natural casing topped with neon green pickle relish, dill pickle spear, tomato slice, pickled sport peppers, yellow mustard, and dash of celery salt on a sesame seed bun.
Damn I'm fuckin hungry now.
I know you say a hot dog should be all-beef and I generally highly respect your culinary taste but I tried some mixed beef & pork Frankfurters and they were really really nice. Specially with some sauteed onions.