Sharpened bolts of steel projected at great speed is a formula for your target to have a very bad day.
Although it was cheaper and quicker to forgo the coal and craft iron bolts, the relentless nature of the undead hordes required a more practical approach. Steel was stronger than iron, and was more likely to put the ravenous undead back in the dirt where they belonged, making it very popular in the last few years of Fellhollow’s resistance.