"So there's still coal in there? Seems pretty odd that they'd pass up on the potential profits."

They might have decided that it was simply more important to have a mine to use for their ritual... or they might be thinking about starting a coal mine fire. Create a fire that is near impossible to put out and renders the surrounding land a smouldering, poisonous, uninhabitable mess for potentially hundreds or even thousands of years? It's what Cortana would do.