The same thing that makes one fiat currency more valuable than another -- the demand/supply/confidence for that currency.
And, further, fiat currencies aren't perfectly liquid with respect to each other.
While an existing fiat currency might derive its confidence from different sources than that of a digital currency (taxes, military, etc. vs. blockchain, mining, contracts, etc.), it does not make them that different. People still declare the coins value on that confidence/demand.