Why do Korean restaurants only serve honey mustard instead of regular mustard?

In many restaurants in Korea, especially those selling burgers or sandwiches, it is difficult to find regular mustard. Regular mustard, which is tasty and low in calories, is a popular sauce among foreigners, so why is honey mustard more commonly used in Korea? Foreigners visiting Korea often say they dislike the sweetness of honey mustard. Regular mustard is also inexpensive, so why is it so rare? Even in foreigner-dense areas like Gangnam, Hongdae, and Itaewon, it is hard to find regular mustard. Is there anyone who can suggest ways to find regular mustard in Korea or recommend other sauces that can substitute for mustard? I need experiences or advice regarding this in Korea.