That’s because the side facing us is reflective; the backside isn’t. And since the moon doesn’t rotate, we always get lit up by it
It rotates once per orbit. Thus, the same side always faces Earth
Just to add a bit to this running theme with another factoid...
The moon “rotates” exactly once per orbit because the attraction from the earth to the moon’s off-center center of gravity has eroded what used to be a much faster rotation.
The moon is slowly doing the same to the Earth, and our days are getting longer. At some point, the earth will also “rotate” in such a way that the Earth show’s only one side to the moon. At that time, there will still be a tide, but it will be essentially permanent.