Shay
explains.
Summary:
The Japanese see octopuses as playful comics or pranksters, not monsters.
The Japanese don't have Christian guilt and fear about sex, and no qualms about erotica or instruction manuals.
The post-WWII Occupation forces, OTOH, *did* have such neuroses, and spread the lack of fun by banning the portrayal of genitalia, leading to (a) fuzzy censored penis hentai and (b) a revival of tentacles.
Plus of course the full-bondage + triple-penetration without obscuring the scene with bodies capability of your friendly little tentacle monster.