Why do electrons rather than protons make up the flow of charge in a metal wire?

Protons are tightly bound to the nucleus of atoms, and the nucleus is

'shielded' from the outside world by the 'cloud' of electrons around it.

It takes a lot of energy to separate a proton from the nucleus. When

that happens, it's a 'radioactive' or a 'nuclear' event, and the atom has

changed into an atom of a different substance. This isn't something

that's happening around us very often.

On the other hand, the electrons are on the outside of the atom, and

it's rather easy to convince an electron to leave its atom and flow off

to somewhere else.