I've always suspected it is a combination of at least two factors. First off, the shotguns were mass-produced items rather than being individual examples of an artistic type. Second, being mass-produced, there is little "romance" associated with the history of their use (unlike, for example, the later cartridge doubles that were shortened for use by stagecoach guards etc). Every longrifle belonged to a frontiersman (even if it never left the township where it was made), every short double cartridge gun was carried to fend off highwaymen (even though most of them were probably shortened after accidents, or in modern times to meet the demand), but those old double muzzleloaders were just farmer's guns (even if they were carried across the continent in a wagon train, or were traded upriver in the late fur trade).
My 2 cents, for whatever it is worth.
Jim