Truth is Siegel and Shuster were trying to make a comic strip character out of Superman, not a comic book one. Comic strips were very popular back in the 1930's and mostly everyone read them. Prince Valiant, Tarzan, Flash Gordon all were comic strips before appearing in comic books. The first comic books were in fact reprints of comic strips! Superman was one of the first characters to appear in a comic book without being a comic strip character first. It was after the comic book took off and the radio show began that he appeared in comic strips! In the early days comic books were in fact looked upon as being for young boys and girls.
Superman has become as popular as he is now because he has appeared in so many mediums: comics, cartoons, movies, TV series, radio, toys.