The external forces acting on an object placed on an inclined plane are gravity pulling straight down, the plane supporting the object pushing at an angle perpendicular to the plane, and the friction between the object and the inclined plane.
The external forces action on a satellite are really just one force, Gravity. According to Newton's 1st law of motion, the satellite will keep moving unless an outside force acts on it. There is no air to provide friction. The only force is gravity which literally makes the satellite fall around the Earth, hence the orbit.