Because the Euro and the US Dollar are legal tender in their respective jurisdictions, thus giving them a competitive edge over other types of money, and — perhaps more importantly — because capital gains are calculated using each currency as a reference in the jurisdiction where it’s legal tender.

So, if you invest in Euros in the US, and the dollar falls relative to the Euro, you owe money to the IRS — and vice verse for the Euro in Euro-countries. It’s difficult for competing currencies to gain traction when you owe money if the legal tender drops in value (relative to the competing currency).