Because Japanese law states that dicks have to be censored, so tentacles aren't censored cuz they're not dicks.

Well Catwoman. Long story short.

Japan's laws forced the hentai makers to censor all male genitalia in their porn.
Don't ask me why they have to censor their porn, I don't know the answer.
But tentacles aren't genitalia, so they could get away with it.
So, after years of making porn with tentacles, it just became a kink.