Why did a U. S. military presence remain In Japan after after the formal occupation ended?

After the formal occupation ended, Japanese government agreed to form an alliance with United States government.

This led to the creation of  The Treaty of Mutual Cooperation and Security between the United States and Japan in 1960. In this treaty, Japanese government agreed to let United States built a military base in its territory in order to gain easier Access in Asia Pacific region.  As per today, we have around 31,000 military personals in Japan.

d: their own individual powers within the new goverment