Matt,
It has a lot to do with how much of the recoil of the gun is attributable to the powder and the expansion of the gases.
For shotguns, that's a very, very small percentage. Think of it, for a shotgun, the shot and the wad weigh upwards of 500 grains, while the powder is 20-ish. Ports can only redirect gases, so for a shotgun, porting is ineffective for recoil reduction because the percentage of recoil attributable to powder is so small.
For rifles and high powered handguns, the powder, and therefore the amount of recoil attributable to the powder and the expansion of the gases, can be a significant percentage of the overall recoil. So it stands to reason that porting, or muzzle brakes, can be very effective reducing recoil for high powered rifles or handguns.