According to The Los Angeles Times, that's just how it is. "Dan Rooney [whose father, Art, founded the franchise] told him to put the decal on the helmet, and he just went ahead and did it and put it on the right side only," said Joe Gordon, the Steeler's former communications director. "Dan was not specific as to whether he wanted it on both sides of the helmet or just the one side. So once it was done, he never changed it."

The article also mentions that a second logo could have been added onto the other side of the Steelers helmets but that the topic never came up in conversation. Although the team does have another logo. The word "Steelers" in black and yellow is also associated with the team. It can also be seen on some merch, the team's website, and it's proudly displayed along with the circular decal.

Although most NFL teams do have their logos on both sides of their helmets, some teams don't have logos at all. The Cleveland Browns are simply known for their particular shade of brown. And the Cincinnati Bengals have helmets covered in black and orange stripes.