There may have been plans to include minable iron ore deposits in the game overworld at one point in development. It is possible those might make it into the game some day, but I wouldn't count on it. Muddy scrap piles can be found even outside of crypts with the use of the Wishbone. Their presence makes iron deposits redundant.

If I were to speculate, I might think that the player could make steel using iron and some other resource. Rather than send the player back to the swamp, a new biome may have iron deposits with greater ore return to supplement a poor exchange rate (e.g. 2 copper + 1 tin = 1 bronze).