A: It's because I hated using the Breaker 12G, even though I liked the gun it was based on. My main issues with the gun include the RoF being awkward and slow, the ammo pickup was AIDs, and it's ammo capacity only gave you three clips.
Q: "Why did you lower the damage that the breaker 12G had?"
A: If you wanted a Breaker 12G that's relatively unchanged, aside from just minor boosts to ammo pickup. Then this might not be for you. As I think the Breaker 12G shouldn't be a direct upgrade from the DB shotgun. Neither should it be overpowered compared to other shotguns. It's better to keep it balanced and to trade off certain aspects you might not actually need.