Because it is further from the Sun than Jupiter and thus has further to travel.
The orbit of Jupiter is closer to Mars' orbit than to Saturn's orbit. Mars = 1.52AU Jupiter = 5.20AU Saturn = 9.54AU Having said that, the positions of the planets are always changing as the planets orbit the sun at different rates. Mars is the closest planet to Jupiter as of February 2011, but this is not always the case.
Pluto orbits the sun, as does Saturn, but Saturn is closer to the sun than Pluto is, so Pluto has much further to go to orbit the sun. Pluto's orbit is longer than Saturn's.
Both Jupiter and Saturn vary their distance from the sun as they move in their orbits. Jupiter ranges from 4.9 to 5.4 AU from the sun Saturn ranges from 9.0 to 10,1 AU from the sun. So at times Jupiter is more than halfway to saturn from the sun and at other times it is less. It would probably be best to say that the orbit of Jupiter is about halfway from the sun to the orbit of Saturn
Planets closer to the sun than Earth orbit the sun in less than one year (Mercury and Venus), while planets further out from the sun than Earth take longer than a year (Mars, Jupiter, Saturn, Uranus and Neptune). The further out you go, the longer it takes to orbit the sun.
Jupiter has a longer year than Earth. It takes about 12 Earth years for Jupiter to complete one orbit around the sun.
Jupiter, Saturn, Uranus, and Neptune have years longer than one Earth year. Jupiter's year is about 12 Earth years, Saturn's is about 29 Earth years, Uranus's is about 84 Earth years, and Neptune's is about 165 Earth years.
Saturn, Uranus and Neptune, here in our solar system. Our techniques for detecting extra-solar planets doesn't work well for planets in distant orbits around other stars.
I ate the Bones
The "Asteroid Belt" is the name we give to the region between the orbits of Mars and Jupiter. Saturn's orbit is about 83% larger than Jupiter's.
Neptune, Uranus, or Saturn.
The planet that orbits the sun closer than Saturn but farther away than Mars is Jupiter. Jupiter is the fifth planet from the Sun and is located between Mars and Saturn in terms of its distance from the Sun.
Mar, Jupiter, Saturn, Uranus and Neptune.