Naoya Inoue had no quarrels whether he will fight inside Japan, in US or in any other country. It's just that organizers and Bob Arum prefer to make the fight happen in Inoue's backyard because they might have already compared the revenues to other places and it's much more suitable for Inoue to challenge Fulton in Japan as that will generate more profits for them.
Inoue can have this fight in a US soil but I guess it is already established that more money can be made in Japan where Inoue is already an icon rather than in US where Fulton is not yet considered in that level.
I agree with that. Based on his records, we can see that most of his fights have taken place in Japan. However, he has also fought outside his country with the same dominant results. So it seems that the revenue generated from his fights matters a lot. In the world of boxing, the higher weight divisions tend to be the money makers, while Inoue's current division may not yet generate as much financial interest.