Because making Iron was essential for survival and also a great way to become rich. The skill of writing not so much when the Palace you used to work in was a ruin and deserted.

The theory goes that a huge volcanic eruption in Indonesia/Flores changed the weather patterns in Europe. This resulted in repeating draughts and famine. The people rioted and civilizations fell. The very complex traderoutes that transported copper and tin was disrupted. Tin and copper was the ingredients of making bronze. This lead to a high pressure to refine the art of turning iron-ore into steel.