Two reasons for this: 

1. Steel has practical value. It can be used to make weapons and armor, which are essential to survival after the Cataclysm.
2.  Because of the Cataclysm, many veins of gold that were previously inaccessible became easy to mine.  Because gold became more commonplace, its value dropped.

However, if good steel is now currency, it can't be used for armor and weapons.  We always assumed there were 2 grades of steel: "noble" ore and "common."  The "common" ore is filled with impurities, and has the same capabilities as bronze,  Whereas "Noble" ore is true, good steel enjoyed by every other game world.  However, it is too valuable to be used as weapons and armor.  A weapon made of noble ore would cost its weight in steel.  So, a longsword made of this should cost 200 Stl.