Answer: Trade
Hey there!
What accounts for the cultural diffusion in East Asia through history is trade.
Trade played a very big role in the cultural, and religious exchanges that happened between Europe and Asia. Through the trade routes, merchants transferred raw materials, luxury goods and foods from areas with surpluses to others where they were in short supply.
The cities located along these trade routes were benefited by this, as they grew acting as international marketplaces for merchants. Historically, Japan became the country which experienced more diffusion of culture from Western nations as compared to other East Asian countries.