Why in a nuclear reactor the neutrons does not interact with the electron of the combustible or the moderator?

The neutron is uncharged, so to first order it does not interact electromagnetically (it has a dipole moment, but dipole interactions aren't going to do a whole lot). The electron is not subject to the strong force. Gravity is irrelevant at that scale. So the only way a neutron and an electron can interact is weakly. And the weak interaction is quite weak--the cross sections for neutron-electron interactions will be small--much smaller than the strong interactions between neutrons and other nucleons.