Why you always see the same face of the moon?

Because the moon's periods of rotation on its axis and of revolution around the Earth

are equal ... 27.32 days.

-- To an observer on Earth, the moon always presents the same face.

-- To an observer on the moon, the Earth is always at nearly the same place in the sky.

. . . . . . If the observer is on the side of the moon that faces Earth, it's ALWAYS in his sky,

and he can watch the Earth rotate completely, roughly every 24 hours 50 minutes.

. . . . . . If the observer is on the side that doesn't face Earth, then it's NEVER in his sky.