What was the main reason for the isolation of Japan?
Japan was not completely isolated under the sakoku policy. It was a system in which strict regulations were applied to commerce and foreign relations by the shogunate, and by certain feudal domains (han). [ The policy stated that the only European influence permitted was the Dutch factory (trading post) at Dejima in Nagasaki. Trade with China was also handled at Nagasaki. Trade with Korea was
limited to the Tsushima Domain (today part of Nagasaki Prefecture). Trade with the Ainu people was limited to the Matsumae Domain in Hokkaido, and trade with the Ryukyu Kingdom took place in Satsuma Domain (present-day Kagoshima Prefecture). Apart from these direct commercial contacts in peripheral provinces, trading countries sent regular missions to the shogun in Edo. wikipedia ]