Cherenkov telescopes for neutrinos do look down for reasons of schielding. Neutrinos can pass the earth, other particles not so much.

But those are a special case, air cherenkov telescopes are looking for "less weird" particles like photons or protons. Those can only be seen looking up, since the primary particles moving down focuses the cherenkov light down in a narrow cone.

The Cherenkov emission is boosted in the direction of travel, i.e. downward. And there are too many lights on the surface of the Earth.