Bronze was used in cannons because it is not as brittle as cast iron. You could therefore forge larger caliber guns without needing a huge amount of material to absorb the impact of the detonation of the firing charge without the breach material failing. Before reinforcing the breach, to make a rifled cannon of large caliber you would have to forge it so large that it would be very restrictive in it's mobility and very expensive in material. But bronze however is rather soft compared to cast iron and IIRC bronze rifling would become malformed and wear down rather quickly.
The solution was to use cheaper cast iron, which is much harder but more brittle than bronze, and reinforce the breach with wrought iron which would better absorb the detonation without the breach material failing.
Dahlgren used an even more scientific approach to the situation and created a smooth transition from the thick breach to the more narrow muzzle, thus reducing anomalies which might weaken the structure of the gun at form transitions and allowing form the impact wave to dissipate more uniformly from the breach.
Actually the rifled cannons should all be black, as they were all painted to prevent the iron from rusting. Bronze didn't need to be painted and I think the army liked polishing them up real nice anyway. Much later they certainly did:
German 37mm Maxim at the Johannesburg Military Museum