why cant electrons be excited by fast moving protons?

I suppose the "electrons" referred by you is "orbital electrons of an atom", not "free electrons".

Any charged particle can give excitation or ionization to an atom (i.e. raise the energy level of an orbital electron or completely "kick-off" an electron from its orbit) if it has high enough energy. Examples are alpha particles, and beta electrons.

A fast moving proton can also cause ionization and excitation to the orbital electrons of an atom. In fact, high energy protons are ionizing radiation. It has been used in "proton therapy" for treatment of cancer patients.