That thing about shark movies only selling well in Japan clearly isn't true, but maybe there's something to it? I'm spit-balling here, but maybe because it's an island nation, most of the population lives close to shorelines which may or may not contain sharks.
Sources:
【#朝ころね】ころねとみんなの夏トーーーーーーク!【朝活】
https://youtu.be/gm1nyq4CmcU?t=9520
[DEBUT STREAM] SHAAAAAARK #hololiveEnglish #holomyth
https://youtu.be/dBK0gKW61NU