The reason that Yugoslavian (and Romanian/Bulgarian too) tour operators used locally registered aircraft was to earn and preserve as much hard currency (Sterling, marks, francs etc) as possible. Communism/socialism wrecks economies, and countries that suffer under that system invariably run out of money, so print more, thereby making their currency worthless. In order to pay for vital imports (like oil) the East European countries pre-1989 ran cheap package holidays for the express purpose of earning Western convertible currency.