In the case of El Salvador, they adopted the US dollar when their native currency collapsed after a bloody 12 year long civil war. Notably the civil war was covertly funded by the United States in order to support regimes aligned with US interests. At the height of the war, the US was funneling $1-2 million USD per day (1980 dollars) to fund an army of child soldiers. [1]

The US is a proximate root cause for the loss of ES’s currency. I could see why they would want an alternative to the USD as legal tender.

[1] https://en.wikipedia.org/wiki/Salvadoran_Civil_War

In general the USD or the euro emerges as defacto form of legal tender on the other side of some sort for societal collapse. It can be a temporary thing, but it is not a healthy thing.