Why The ionosphere can only bend signals back to Earth up to a certain maximum usable frequency, which even under the best conditions rarely reaches much above 50 MHz. UHF signals at 430 MHz and up simply pass straight through the ionized layers into space, so there is no skip to return them beyond the horizon. That leaves UHF working essentially line of sight, extending only slightly past the visual horizon because of a small amount of tropospheric refraction.
Watch out The idea that they are too weak is the common trap: power is not the issue, since a high-power UHF signal still will not bend back to Earth. D region absorption is a real effect but it attenuates low HF signals like 160 and 80 meters during the day, not UHF.
UHF goes through the ionosphere, not off it. Above the MUF means no skip, just line of sight.