GPS satellites are placed on six different orbital planes, which are angled at 55 degrees from the equator. Therefore, satellites don't pass over the North or South poles and that is why no satellites are recorded by SkyView™ on those dark areas.
GNSS satellites are placed on different orbital planes, which are angled between 55 and 65 degrees from the equator. Therefore, satellites don't pass directly over the North or South poles, which creates the dark (no satellites) areas on the north or south parts of the polar graphs. Keep in mind that the Earth is also rotating.
The position of the "no satellite" zone depends on the location of the antenna being tested. SkyView™ users checking antenna reception patterns and quality on the northern hemisphere, will see the no satellite zone at the top of the polar graph (north). The example below is from Fremont, California USA (Latitude: 37 degrees north).
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Users checking antenna reception patterns and quality on the southern hemisphere, will see the no satellite zone at the bottom of the polar graph (south). The example below is from Santiago, Chile (Latitude: 33 degrees south).
Users running SkyView antenna coverage test at higher latitudes (closer to either pole) would see the "no satellite" circle closer to the center (90º, zenith) of the polar graph.