It rarely has anything to do with the legalities, and more to do with not knowing shit about porn production, or underestimating the market for Western-produced porn from Japan. Most of the bozos that post here and elsewhere basically want to fuck Japanese girls and think they can mint gold with their dick and don't know anything at all. There's been two companies that I've seen that have actually produced real porn in Japan.
One went nowhere after producing a few scenes where the guy didn't know how to fuck and the girl looked like she was in a ton of pain because apparently there was neither the time for proper foreplay or the budget for lube.
The other was Little Mango, which had very good production values, hired a number of actual Japanese porn stars (one of whom is an ATF of mine, Yui Komine) and filmed them getting fucked by a white guy, released 4 or 5 movies, and still couldn't make any money probably because porn on the internet is free. They did a Kickstarter campaign to do another movie and hire a more widely known porn star, and they couldn't even get 50% of the money they wanted pledged. That was the last I heard of Little Mango.