Why is elementary school 6 years (middle and high school 3 years, college 4 years)
The only answer to the question of why is that it was decided so. In other words, the fact that elementary school is six years, junior high school and high school are three years, and college is four years, were all decided by human beings at a certain time.
The school system in Japan was established in 1947. The School Education Law was enacted in that year, and it established the school system.
Before that, elementary school sometimes lasted eight years, and sometimes three or four years.
