The hard hammer springs must be a carryover from flintlock days when the hammer was extra heavy, laden with the flint, and had to overcome friction with the frizzen to fire. Modern shotgun primers are not that hard to ignite, evidence of that are the soft springs in autos and pumps, you can cock them with your little finger and they fire OK even though they push much longer and heavier firing pins and springs.
As a matter of fact, in a flintlock gun the spring doesn't have to be extra strong - perhaps it was necessary to ensure quick and positive ignition in a shotgun, but ignition in principle could be achieved without much strenth. "Sibirka", an ancient flintlock rifle design made for Siberian fur hunters, featured mainspring made of a long piece of horn - and they worked.
