What is the reason why there are huge craters on the Moon

In the beginning stages of the solar system forming, all of the celestial bodies of the solar system which are the moons, the planets and others were hit with debris, this is what caused the moon to have such big craters, the reason for this is because the moon doesn't really have an atmosphere to protect it from the asteroids. It is true that the atmosphere of a planet is what protects the planet from being hit with meteorites and asteroids. The bigger the atmosphere, the more protected the planet or body is from being hit with debris. Also the moon is geologically dead, meaning that it can't erase the crater, on Earth, there are craters which have been removed by the Earth naturally. It is also true that the Earth is hit by meteorites and other bodies everyday but the atmosphere of the Earth is what filters all of that debris out, if the meteorite or asteroid is big enough, it will penetrate the Earth's atmosphere and impact the surface of the Earth. An asteroid or body that hits the Earth needs to be about one hundred feet in size for it to make it through the atmosphere of the Earth.

Craters are the result of large asteroids/comets impacting the surface of the Moon. Back when the solar system was young, there were many more collisions taking place than there are now. Because the Moon has no atmosphere, & therefore no weather to erode or disturb the craters, they remain visible to this day.

These are the scars left over from an earlier time in the Solar System's history when there were a lot more large asteroids. All the planets are covered with such scars, but 3 billion years of subsequent geological and weather action have mostly erased them. Even so, 157 meteor craters have been identified on Earth. It's worth visiting the ones nearest to you; it will give you a greater appreciation of the Moon's surface. The site below catalogs all known meteor craters on Earth and tells you how to find them.

during the heavy bombardment stage in the early life of the solar system, all the large satelites, moons and planets were hit with debris.

most if not all the craters can be attributed to meteor/ comet and other space debris impacts

It was made By the falling of very huge celestial bodies during the early years of the solar system on different planets and even on the moon .....

Many of outer space objects/bodies (asteroids, meteors, etc.) have collided with moon, causing many deep craters on Moon's surface.