Reason being that in Spanish "El" is an article which translates to English as "the" so the translation of El Salvador is literally "THE SAVIOUR". So if you say to someone "my friend is El Savadorean" you are essentially calling them "the one and only citizen of El Salvador". For that reason a person from El Salvador is either "Salvadorean" or "Salvadorian". Now some people refer to the people of El Salvador as Salvadoran, so there is some dispute there, but El Salvadorean is incorrect.
If others think otherwise I invite some discussion on this topic.