Most currencies aren't pegged either, it's just supply & demand - with the caveat that people will tend to arbitrage based on interest rates (as @TW mentioned) which tends to keep them roughly correlated.
Since crypto currencies mostly have no underlying interest rate to arbitrage, it's just supply and demand (and they're relatively high-friction, so triangular arbitrage is maybe less effective at keeping everything correlated).
Most currencies aren't pegged either, it's just supply & demand - with the caveat that people will tend to arbitrage based on interest rates (as @TW mentioned) which tends to keep them roughly correlated.
Since crypto currencies mostly have no underlying interest rate to arbitrage, it's just supply and demand (and they're relatively high-friction, so triangular arbitrage is maybe less effective at keeping everything correlated).