Answer: A geostationary satellite stays over the equator and its signals cannot reach the polar regions properly.
A geostationary satellite revolves in the equatorial plane with the same period as the earth, so it appears fixed above one point on the equator. Its orbit is about 35,780 km above the equator, and the curvature of the earth blocks its view of the far north and south; the signals also have to travel a long, slanting path to reach the poles. So polar regions are studied with satellites in polar (sun-synchronous) orbits instead.