The reason is that the molecules of glass naturally resonate at UV frequencies, so that there is much motion, causing heat generation, not transmission of light. Visible light does not resonate with glass molecules so the forced vibrations do not cause large amplitudes. Less heat is therefore generated and more light transmission occurs. Light moves slower in glass. Actually, light moves between the atoms as fast as in a vacuum, because that space is a vacuum.
Longer waves vibrate entire atoms and molecules which is kinetic energy or sensible heat.
Ex.5 Sound requires a physical medium in which to travel. Light does not. We distinguish between something being diluted and something being annihilated. In accord with the inverse-square law, light intensity gets weaker with distance, but the total amount of light over a spherical surface is the same at all distances from the source.