I just assumed so because once I move from Texas getting mustard on your burger was as rare hen’s teeth. A quick Google search seems to confirm it.
Article in the Atlantic:
And guess which state in America uses mustard on its burgers, to the exclusion of ketchup? No, not some liberal, elitist state like Taxachusetts. It’s Texas.
City-Data:
Krystal down here (Southern version of White Castle) only serves their burgers with mustard and a pickle.
The Baltimore Sun:
What’s with mustard on burgers? Everyone knows mustard is for hot dogs and ketchup is for hamburgers. Maybe it’s a southern thing because I moved here from New York 18 months ago…