Who says we don't always use it?
Power in an electric circuit is Watts, which is current in Amperes times voltage. Amperes are one Coulomb or 6.241509x10^18 electric charges per second flowing through a conductor. So a fixed amount of power (Watts) moves a known amount of charges. If we were sometimes moving protons instead of electrons, maybe we’d notice three orders of magnitude difference in quantity of charges in different experiments?
This also is why electric power flows along a wire at the speed of light, while electrons can only travel along a wire at the speed of a snail, or about 1 mm per second
One doesn't follow from the other. We can easily transport power by making things like a chain or a fluid move back and forth, without any net travel. In a setup with a loudspeaker and a microphone as just one example the air transfers energy from one to the other without any net movement. In those cases it's clearly the movement itself which transfers the energy. Therefore energy transport by AC is no proof for the need of an electric field for energy transport.
That's not say to there is no electric field, or to deny its role in power transfer. There certainly is an electric field. But that field is intimately tied to the electrons in the conductor, and power transfer is intimately tied to movement of those electrons and the way electrons repel each other stronger when they get closer together (or other charge carriers, but in typical conductors that means electrons). You can't have one without the other.
Still though, you asked how we know electricity uses electrons rather than protons and I'm sticking with "they're 2000x different in mass, there would be some measurable difference between them" even in an AC circuit. Oscillating a more massive conveyor belt back and forth 'in-place' is harder work than oscillating a lower mass belt. "Electrically charged" means "interacts with the electric field" and if energy in the electric field is moving heavier protons back and forth, wouldn't that be distinguishable from moving lighter electrons back and forth? Slower movement of protons, more heat generated, something like that?
