Because the farther a body is from the sun . . .-- the longer it has to travel to cover one revolution ... we learn that from Geometry ... and-- the slower it moves in its orbit ... we learn that...
Rotation is a spinning around yourself (i.e., around an axis) ; Revolution is going around something else (the moon around the Earth, the Earth around the Sun).
Rotation is spinning on the objects axis and revolution is orbiting around an external axis. Rotation determines night and day and orbiting determines the seasons and the years.