It's because the noble metals are the most suitable thing for use as money on earth, and of those noble metals, gold was the best choice since it doesn't react with oxygen.  Another noble metal is then needed to increase granularity of gold to prevent counter party risk, and silver is the only good candidate there.  

This means there is a functional Schelling point for gold and silver as money, while in cryptocurrency there is actually a REVERSE Schelling point where people are incentivized to branch out and create new ones in order to try and get windfall early adopter profit.  Nobody is able to just walk out and invent a new noble metal, so this doesn't happen there and the Schelling point of gold and silver is preserved.