The gravity on the Moon is less than the gravity on Earth primarily because the Moon has a much smaller mass and a smaller radius compared to Earth. Gravity is determined by the mass of an object and the distance from the center of that object.
The mass of the Moon is about 1/6th that of Earth, meaning it has much less gravitational pull. Additionally, the Moon has a smaller radius, which means that the distance from its center to any point on its surface is shorter compared to the distance from the center of the Earth to any point on its surface. Since gravity weakens with distance, the combination of lower mass and smaller radius on the Moon results in a weaker gravitational force.
To put it simply, the Moon's gravity is weaker because it is a smaller and less massive celestial body than Earth.