Because the Chaingun is already the mod's Super Shotgun, the Shotgun is already more powerful than vanilla, and because two pump shotguns would feel redundant.
Don't get me wrong, I love these guns a lot. The only thing I feel could be an issue is that, since the Chaingun takes the SSG's spawn, the Shotgun tends to not run out of ammo while you might not get a lot of use from the Chaingun.
