Law of supply and demand.
Bronze is mainly from copper and tin. Main producers are Chile, Peru and China.
Stainless steel is mainly from iron, nickel and chromium.
Copper have better thermal conductivity so it is widely used by other industries as well such as wires, etc. not sure how much the demand spread is, I assume those industries requires more of copper more than us need it for valve body and trim.
Aside from small sizes bronze valves for utilities application (which can be found in most hardware store), I don’t think there are so many player who made good quality bronze valve for medium and/or bigger sizes. Maybe that is also a factor.