Why did nations usually choose to create republics after the fall of multiple empires after WWI?*

The end of WWI left many nations unhappy. The Germans were humiliated and lost territory and had war payments. The winning countries such as Japan and Italy did not gain enough land, but the Treaty of Versailles caused the war.

the answer to your question is:

strong-mayor system

cultural/diplomatic relations. most medieval japanese   culture has chinese origins. writing, architecture etc.