Because stablecoins have a security vulnerability, they're pegged to assets who's value can be manipulated by a third party. I'd imagine there are many motivations to El Salvador diversifying their currency market, remittances might be one, but shelter from Fed monetary policy is a strong additional benefit.

Also, stablecoins that are pegged algorithmically rather than by 1:1 hard asset backing will eventually lose peg. How long that takes on average depends on the design of the system. Look at Nubits and then look at MakerDAO to see different examples of algorithmically pegged stablecoins.