Simple. Any weapon that grants essentially infinite ammo for skilled precision should only be reserved for an Exotic weapon, and it already has: The Ice Breaker. For a Guardian who's skilled with precision shots, there is almost zero downsides for using the Black Hammer to its fullest potential. There needs to be a downside. No reload is good enough. Infinite ammo is not. If the Ice Breaker and Black Hammer both attacked Valus Ta'arc, the Black Hammer would still outdamage the Ice Breaker in a given minute, assuming all shots are precision shots.