Question

why we see only one side of the moon

Answer 1

This is because the moon rotates about its spin axis at the same rate that the moon orbits the earth this condition is termed as synchoronus

Answer 2

Only one side of the Moon is visible from Earth because the Moon rotates about its spin axis at the same rate that the Moon orbits the Earth, a situation known as synchronous rotation or tidal locking. The Moon is directly illuminated by the Sun, and the cyclically varying viewing conditions cause the lunar phases.