Because fire.

The Earth Empire would beat the Fire Nation. Let me ask you, which is stronger: Comet enhanced firebending or spirit energy cannon?

Army of comet enhanced firebending vs 1 canon is more accurate

One spirit energy cannon on a GIANT MECHA SUIT!

Key word one.

The mech was overrun by a small team of benders right? Now imagine an entire nation coming after it. Also the cannon needs fuel. Cut off the supply lines and the cannon becomes useless.

Spirit energy.

And to get inside they would need technology they simply don't have.

What would they need? And again who says they need to get inside? Like I said without supplies the mech cant run. Cut off the supply lines and it's over.

They could also knock it down or superheat the mech until the people inside die of heat exhaustion or because of them just gettin cooked.

Spirit energy means they don't need supplies. They have everything they need inside the mech.

And fire will not get through this armor.

Seeing that the Fire Nation was beating up the Earth Kingdom for 100 years, I'd say the Fire Nation.