Xenophobia and belief in Japanese racial superiority:

https://en.wikipedia.org/wiki/Nihonjinron#Specific_theses

https://en.wikipedia.org/wiki/Ethnic_issues_in_Japan#Pre-war...

Much of it can be blamed on propaganda driven by economic decline and political scapegoating. While Korean music, drama, and movies are popular in Japan, consumers are mostly women. Among small but growing and very vocal population of Japanese men, Korea-bashing books and manga are popular. Later trend is not unrelated to continuing economic decline of Japan. And Japanese politicians are leveraging and fueling that trend to divert the blame.

Maybe because they're not Japanese enough.

See https://www.upi.com/TOP_NEWS/WORLD-NEWS/2019/04/17/ANTI-KORE...