Cast iron has a melting point of 1250+ °C (which was hard to achieve with ancient bellows) while copper have 1085°C and brass 900-950°C (depending on the composition of the alloy), moreover as said previously cast iron needs to be heat treated after pouring to ensure workability and toughness. Charcoal wasn't cheap even in those times, so would have been a double waste, the work of a person to make a tsuba from scratch was probably cheaper.