Because they died? A lot of people die and don't get called heroes.
You raise an interesting question here.
According to my dictionary, a hero is "a man distinguished by exceptional courage, nobility, fortitude, etc." I think astronauts are characterized by both exceptional courage and exceptional fortitude.
Actually, I think there are millions of Americans who fit this definition; in fact, I believe the American nation is a heroic one. (I'm not an American myself, so this is not a self-congratulatory opinion.)