Question

why does only one side of the moon always face the Earth?

Answer 1

Your question has a fundamental error. The is no part of the moon that never faces the sun. I suppose you could get very picayune and argue that the poles don’t face the sun but even they are lit up at one time or another. 

Answer 2

Because the rotational and revolutionary speed of the moon is approximately equal. Therefore when moon's position is towards us we can see it's one side only but when it revolves around the earth it also rotates on its axis with the same speed thus at another position also we can only see it's one side only. For more clear information you can check out YouTube video related to it.