From what I understand MK isn't necessarily banned in Japan so much as physical retailers either aren't allowed or refuse to sell it due to it's gore factor. I know you can buy it easily digitally over there, and it has a niche community, but that's just it. It's niche. And while, say, Banjo-Kazooie also is niche in Japan it had the benefit of already being heavily associated with Nintendo and isn't an uphill battle for family friendliness. At a first glance I would argue most average families would say Metal Gear, Tekken, and Persona don't straight up look inappropriate unless you're actively following along with them, whereas MK with it's liberal use of blood and guts is immediately dismissed as trash by the older generation.