Why do the inner planets have fewer moons?

Answer 1

The inner terrestrial planets are far less massive than the giant outer planets and therefore have weaker gravity.

Answer 2

The inner planets have fewer moons because they formed closer to the Sun where the gravitational pull was strong enough to prevent significant amounts of material from coalescing into moons. Additionally, the inner planets' gravity is not strong enough to capture passing objects as moons like the gas giants, which are located farther out in the solar system.