The only real reason is snob appeal!!
In many of the "classic" shotguns the 28-gauges were built on the same frames as the 20-gauge and are heavier. The Winchester Model 12s and 21s come to mind immediately. Also, in the Browning Superposed the 20-gauge was available in a Lightning model, but the 28-gauges and .410-bores were only offered in Standard weight.
When Parker Bros. salesmen started pushing the 28-gauge, the guns from Meriden were rebarreled 0-frame 20-gauges that had been languishing in inventory and someone got the bright idea that that would be a way to move some product. Some years later they added the 00-frame.
Ithaca Gun Co. probably made the lightest 28-gauges in their Flues Model with some as light a 4 3/4 pounds. How many of us can actually shoot well with a gun that light? I know I do much better with guns in the 6 1/4 to 7 pound range.
While the "standard" 28-gauge shell 85 to 115 years ago was 2 1/2 inch case with 1 3/4 drams of bulk smokeless powder pushing 5/8 ounce of shot, there were 2 7/8 inch NPEs available. Gun cranks like Chas. Askins had a 30-inch barrel 6 3/4 pound Parker Bros. and was handloading those 2 7/8 inch cases with up to 2 1/8 drams of bulk smokeless and 3/4 ounce of shot!!
