28 ga has been associated with Upland Birds and Skeet since - forever. It fills a niche no other gauge can.
First off the gun itself is a marvel to tote, and still retains the abilty to mount to face sooo smooth, pick up the bird and follow thru. NOT whippy, not like a hunk of 2X4 - instead - with minimal, graceful efficencey of form.
Second - the payload to bore is unique, magical if you will.
Even in heavier guns used for skeet - well the best scores in skeet are posted in the 28 ga listings.
The 12 ga is the most versatile guage, even more now-a-days with all the loadings and recipes avail. 20 ga is next. .410 will always be the one bringing up the rear. It is not as effecient. It is better with today's loadings, forcing cones and such.
I have MY own other thoughts in regard to Slugs, and larger shot sizes . One could say with the payload to bore - "if it ain't broke- don't fix it " Kinda like old man Bulova " I can teach a person to fix a watch - I can't teach him when to quit fixing it". some things are best left alone. Too many offerings for less money in 12 or 20 - avail a lot more places.
Not a lot folks use the 28 ga, so that is a cost / marketing / condsideration - folks that do use them - reload - if they shoot a lot. Others use for traditional reasons - the Plantation Quail hunts for instance. One does NOT show up with a guage larger than 28. One uses the .410, 24, 32 or 28 ga.
There "may" be some folks that have researched, "developed" and actually tested slugs, and other loading for the 28 ga...time will tell perhaps -
Someday.
