And Japan would want to become part of the USA why?
Also - the USA would want them in why?
For these all the reasons are very similar: further economic and military integration.
So, from the American perspective, the switched out Japan greatly reduces our ability to reach the Pacific Ocean, and even with good relations with them, routing through Japan to reach the Pacific means they control those routes and can cut them off at a moment's notice, and the northern route through Washington State is simply not going to be able to handle the sheer amount.
On the Japanese side of things, the US is already one of Japan's largest trading partners the only reason we're not #1 has to do with geography. Put Japan where California is and the US will be their #1 trading partner. Full political integration with the US means no longer having to deal with international trade negotiations, they'd just be part of the common marketplace. That would have SERIOUS benefits for their major corporations.
Secondly on the military front, the only reason the US tolerates Canada not being part of the US is because Canada is so sparsely populated, so culturally similar, and it's entire nation so, well, vulnerable to the US that Canada cannot not be in the US sphere of military influence and control. Much of US defense doctrine is to make people fight OVER THERE, and keep the fighting away from the North American continent. Mexico is in a similar position, but there's also some serious geographical defenses in the southern US that allow Mexico a bit more leeway in not being as beholden to the US.
Japan being where it is now means that the US has, in effect, lost control of one of the coasts of North America. That is a militarily... unacceptable position for the US to be in. Japan also has a large enough population and economy that they could choose to do their own thing unbeholden to the US and thus prove a dangerous beachhead for hostile foreign powers if they so chose, and there's not much the US could do about it. As such, fully integrating Japan as formally part of the US solves that issue and resecures the west coast of North America.
So there ARE reasons for both to see advantages to integration. On the other hand, there's some serious obstacles too. There's MASSIVE cultural differences between the US and Japan on multiple levels. Japan's laws on many subjects would be in violation of the US constitution and laws, and I can see a lot of Japanese folks being hesitant to, well, have to adopt the more liberal American Constitution.
I think in the long term such integration would happen though. For a few reasons, the first is Japan's baby bust and need to support an aging population. Secondly you'd see massive cultural exchange happening even more than it does now, which over time would break down the barriers. Finally, the economic and military advantages to integration would only grow over time, especially the economic ones, as the two economies become more and more entwined not having to worry about international trade law and treaties being negotiated and renegotiated between DC and Tokyo would been seen as highly favorable to corps in both the US and Japan.