Thanks, this is indeed the question. Thinking out loud: the coverage is probably somewhat conic therefore if you want to scan the ground or lower atmosphere an high altitude is optimal, while scanning the upper atmosphere could be done from the ground.
Perhaps earth's spherical shape gives an advantage to the satellites in both cases ?
Maybe, though a GEO satellite (or really any satellite) will always be much much farther from even the upper atmosphere than the ground will be, so satellites have a pretty dominant coverage advantage.