Fertikeen as the material for hardeners was decided upon by Lancake after he asked some factions what their resource usage was like. Most people were using very little fertikeen, since it's only used in niche effects and pulse weaponry, which requires relatively few modules, so it was chosen as the material for hardeners.
If you make hardeners craftable out of any ore, this will get in the way when the universe gets unequal resource distribution.