Ted, there are various reasons for making a sidelock in addition to the intercepting sear. One is the ease of accessing the "guts" of the gun, especially if the locks are hand detachable. Another is leaving more "canvas" on which the engraver can do his thing (although that's also true of sideplated boxlocks). Another is that--at least per the British, who contributed so much to the development of the modern sxs--the sidelock is a superior design. And there are those who will contend that in general, you get better trigger pulls with a sidelock vs a boxlock.
But our gun industry focused on numbers with only a tiny percentage of their output represented by the "best" guns they made. In contrast, there were several British makers who made their names based on a much smaller number of "best" guns. So a very different approach to the business. In this country, no one ever adopted the business model of "best" guns only. Or mostly.
