They were the new style - gotta' have the newest. They tend to be tough, what with the thicker barrels and through-bolt stocks. Sighting over a single barrel must have been perceived as the better way to do things (yeah, I know, don't focus on the barrels, yadda yadda). And, yes, firepower had to be a (the?) big seller. Imagine how many magazine guns would have been sold in the last century if they only offered one in the chamber and one in the tube?