There were many advantages to refining in space, especially for minerals mined from space. Transporting the unprocessed ore to the surface through the space elevator meant transporting the useless slag back. The price of untreated ore wasn't high in itself, and including the transportation cost of the space elevator, it would be difficult to be price competitive in the international market.
Although the cost of transporting through the space elevator was low, it was still a cost.
Future Mining owned a smeltery for the process of treating ores, and there was no need to profit from selling unprocessed ore. Only processing ore into finished steel products could maximize profits.