Actually, I'd say Spielberg was best known for Jaws at the time, which after Star Wars, was the biggest movie of all time, not adjusting for inflation. Lucas' success may have been in sci-fi, but the Star Wars movies were very mainstream. While Ford may not have had a record as a leading man, he was still very popular after Star Wars and Empire Strikes Back and combined with the creators of the 2 highest grossing movies of all time, made everyone aware of Raiders before it even opened. And it opened at #1...something that movies "coming out of nowhere" don't accomplish.