When Pittsburgh's NFL team changed its name from the Pirates to the Steelers prior to the 1940 season, there were no logos on player's helmets. It wasn't until the 1950s that designs on the sides of helmets became popular. Like much of the league, the Steelers' first design was to simply add a player's number to one side of their gold helmet.
In 1962, the Cleveland Steel Company suggested that the Pittsburgh Steelers adapt the same logo that U.S. Steel had designed for the American Iron and Steel Institute. The steelmark logo has been created to market steel to consumers and was the centerpiece of a large advertising campaigns throughout the 1950s and beyond. U.S. Steel said "steel lightens your work, brightens your leisure and widens your world," hence the three, multi-colored hypocycloids on the logo.
(Later on, some marketing folks decided to assign new meanings to the three diamonds. Each was supposed to represent the three materials used to produce steel -- yellow for coal, orange for ore and blue for steel scrap.)
Before the Steelers were able to gain permission to use the logo, the team had to gain permission to change the word "Steel" to "Steelers." Initially, the team put the logo on one side of its gold helmets in order to test out the look for the year. When the '62 Steelers finished 9-5, the team decided to keep the one-sided logo and switch the helmet from gold to black (so it would stand out more). Almost 50 years later, the logo hasn't been changed.