Good question.
Jupiter has a lot more mass than either Mercury or Venus, so you would expect it to collect more moons for that reason. However, Earth is only slightly more massive than Venus, and yet Earth has a quite large moon, and similarly Pluto is even less massive than Mercury, but also has several moons. Part of it is probably just the chance of where objects were in the solar system:

Earth's moon came about as the result of a huge impact in the distant past. Pluto is in the Kuiper belt, far enough from the sun that all of the little comets haven't been swept out of orbit by planets, so Pluto is able to collect some. Why, then, does Mars have moons? Probably chance.