The best explanation for why the Moon's surface gravity is only one-sixth of Earth's surface gravity is:
Gravity is the result of an object's mass and size; the Earth is much larger than the moon.
This response highlights that gravity depends on both the mass of an object and the distance from its center. Since the Earth has significantly more mass than the Moon, its gravitational pull is much stronger, resulting in the Moon having only one-sixth the gravity of Earth.