China didn't rule japan because it's separated by sea from the
mainland, because China was the only power in the known world,
Japan copied most chinese culture and language
Korea was a vassel state of China until mid 19th when Japan took it
over from china. Korean writing were only invented like 700 years
ago, for the convenience of low class people. The elites still
spoke and wrote in chinese. Most korean history books and important
legal documents were written in chinese.
Similar story happend to vietnam. China ruled Vietnam directly for
over 1,000 years, vietnam was made a province bearing the name An
Nan (the Pacified South). Later viets rebelled and drove the
chinese out. but the culture remained. Again their history were
recored in chinese and the ruling class wrote in chinese and their
school textbooks were Confucius's books same as in china. Then the
french came in 18th century and colonized vietnam, and created a
latinized writing of vietnamese.
After WWII with the rise of nationalism and cold war, both korea
and vietnam completey switched to the local languages, but
thousands years of cultural adoption is much harder to rid
of.
think about: a hundred years later, someone might wonder about a
similiar qustion: why do singaproean speak english?
LOL