Why is some planet don't have moon?

Answer 1

It has to do with the formation of the planet and happenstance. The Earth has a gigantic moon relative to the Earth's size because (it is currently theorized) when the Earth was nearly formed, it collided with another planet (Orpheus) at an oblique angle which scattered debris around the forming Earth. This debris coalesced into what is now our Moon.

Mars, Venus and Mercury did not have these types of collisions so they did not have a moon form when they were formed.

However, Mars' gravitational pull has captured its moons: Phobos and Deimos. These rocks that look like potatoes are thought to be captured from the asteroid belt.

The Jovian Planets, Jupiter, Saturn, Uranus, Neptune all have multiple moons. Most of the Jovian moons are thought to have been formed at the same time as the planets during the accretion phase of the solar system. Some Jovian moons are small captured rocks from the asteroid belt or the Kuiper Belt (the Kuiper Belt is a ring of rocks and ice left over from the formation of the solar system which revolves around the sun outside the orbit of Neptune.) The largest captured moon is thought to be Neptune's moon Triton. This moon revolves around Neptune in a "retrograde" orbit. This means the moon goes around Neptune in the opposite direction in which Neptune spins. It is about 3/4 the size of the Earth's moon and could have been a planet in its own right. But it was captured by Neptune and is doomed to crash into the planet or be torn apart by tidal forces creating a new ring system around Neptune in a few million years. Neptune has 4 known moons that are thought to be captured Kuiper Belt objects

Pluto, no longer considered a "planet" rather a "Dwarf Planet", has a moon, Charon. It is thought that both Pluto and Charon are two individually formed Kuiper Belt chunks of ice and dust that are caught in a gravitational dance.

Answer 2

Some planets do not have moons because they may not have acquired one during their formation process. Factors such as gravitational interactions with other celestial bodies could have prevented a moon from forming around a planet. Additionally, some smaller planets may not have enough mass to capture a moon into orbit.