BECAUSE there are ELECTRONS in the OUTER SHELLS of ATOMIC STRUCTURE, and PHOTONS are the EM force carrying BOSON. It would be impossible for a photon not to interact with the first few layers of atoms in a manner which would keep it from penetrating any further, other than to be absorbed and increase the temperature of the mountain. A NEUTRINO, on the other hand, ONLY INTERACTS WITH THE ELECTROWEAK part of ATOMIC STRUCTURE (a MUCH smaller target), AND SIMPLY CHANGES MODE OF OSCILLATION. Unless it encounters a nucleus of something like a chlorine atom (which it will change into argon), it just keeps on going. Or did you have some other explanation in connection with an aether?