Why are chopsticks the traditional eating utensil in japan?

Answer 1

Japanese people use chopsticks to eat in large measure because it's convenient: much food in Japan is served in small pieces that can easily be conveyed one by one to the mouth with chopsticks. Hot food can also be cooled with the breath more efficiently when it's held with chopsticks than when held with a spoon. (Most chopsticks used in Japan are wood and so do not conduct heat well.)

That said, young children generally do not use chopsticks to eat with: spoons and forks are more common.

Likewise, both Asian and Western style spoons are in common use in Japan, as are knives and forks (and, to a lesser extent, other cutlery); it is common to eat, for example, spaghetti or steak using the same implements used in other countries.

Answer 2

Chopsticks are the traditional utensils because they adopted them from China. They had no influence from western civilization at the time, so it was natural to take in things from China.