Because they are in orbit around the Sun and the ones closest to the Sun (such as Mercury) where the gravitational force is greatest have to move the fastest to keep from falling into the Sun.
The gravitational pull from the sun and other planets and since our solar system isn't in an exact O they move at different speeds. and also to sustain life :P
Thanks guys, you really helped my Science assignment :P
Google, however, failed :D
why dont you tell us the answer instead of acting like a snob.
gravitational pull of the sun
wrong but close.