Well, the pickaxes requires the players to reach a certain level to use them, along with the slightly higher price tag between each type of pickaxes. Second, the durability of the pickaxe that you use in the Dangerous Mines does not decrease, in case you didn't realize. The Ol'West team most likely didn't want to add in the other pickaxes other than the wooden pickaxe because it would have made mining in the Dangerous Mines easier by reducing the amount of hits needed to break the mine walls along with giving out ores that can only be obtained from higher levels in the regular Mine, resulting in higher amount of gralats obtained in a short amount of time, while the pickaxes used remains intact.