I think it's the 'equalizer' aspect. A knight would have to train for years to be proficient in swordplay, horse riding, and usage of a lance. Longbowman had to train from childhood. A wizard has to study magic for years.
A peasant could pick up a gun and theoretically kill any of them.
Americans in general are also more familiar with guns than swords or bows, so the "effectiveness" is more intuitive for our minds.