I think it is because korokke is "just" a Japanese version of the Western croquette, therefore, not "original Japanese food".
I do not think that people would pay and eat korokkes in a Japanese restaurant in a foreign country if the guests can choose among dishes such as soba, sukiyaki, sushi, sashimi, teppanyaki and the sorts.
Korokkes are very tasty (I love it with ketchup), but it is not an original Japanese dish compared to the ones we know.