Why always conservatives.
If you will allow me to completely talk out my ass about this, with no real in depth knowledge or expertise on the matter....
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I happen to read a rather pro-Japanese newspaper over there. In Japan's case, it is my opinion that the conservative party in power right now has held one objective for 50 years: The military autonomy of Japan. However, the Japanese public at large likes the security of having the US troops occupy their country. Particularly, in Okinawa, where the US troops literally saved their lives and turned their oppressed, impoverished, and starving island into a prosperous community. Public support has not been in favor of re-militarizing Japan.
I think, for these reasons, Japanese conservative PR/media have been focused on accomplishing a few tasks. First and foremost, portraying their East Asian neighbors as aggressors who continue to execute illegal provocative military action against Japanese territorial claims. With South Korea illegally occupying Japanese territory right now and China repeatedly disrespecting Japanese Naval territory, that's an easy case to make. The conservative papers, thus, have been full of articles demonizing Korea and China militarily and politically, but there also have been a lot of articles about the public at large having anti-Japanese sentiments with a warlike tone.
Another objective is emphasizing the strengthening of the ties between US and Japan. I suppose this is to reassure the public that a Japan with military autonomy would not be vulnerable, but rather, would be a force that could better protect the shared interests of US and Japan in that part of the world and abroad. I've come to expect generally favorable portrayal of the US. In the last few years, I've seen a lot of articles emphasizing violent acts of Islamic extremism, and since the death of two nationals at the hands of ISIS, the story really has legs.
With all this considered, I'd say a museum that shows Japan being aggressive against East Asia would not be good for them when it comes to swinging public opinion in favor of a full-scale independent Japanese military. They can't have public going "well, last time we had a military...." and seeing what good it did them.
On a side note, I wonder if Japanese people know about the Manchurian Incident.
