Steel was a high ticket item before electric blast furnaces. It melts at a very high temperature and had to be forged in small sizes, not melted in huge crucibles and poured out as is done now. You can't melt iron or steel in a conventional oven or forge using wood or coal. It requires coke (or electricity) to get the temperature hot enough.
Brass melts at 1,700º F +/-
Iron is 2,786º F
Steel is about the same, but requires a mix of several metals and carbon.
It's one thing to forge a long, straight piece like a trigger bow in iron (which can then be filed thin), or wrap a thimble around a mandrel, but a 1/16" thick flat patchbox lid made of steel would be a lot of work for a 1750 gunsmith.