The choice was natural since Superman is considered the greatest superhero in history. First introduced in Action Comics #1 in 1938, the character gained immediate popularity with young readers. In fact, the character provided the template for what is generally called the 'superhero' now - a flying, super-strong character wearing a skin-tight outfit and a cape. Superman continues to be popular in comics, toys, and film today.
The example at right is shown on a 'Kresge card' - a packaging exclusive to the chain of retailers that would later become known as K-Mart.