For me it is the same attraction as a Harley Davidson Motorcycle. Lots of shiney: shiney paint, shiney chrome, shiney windshield, big stout, heavy. The British 20 bore was meant to shoot a 3/4 oz load for it's daily diet even though it was typically proofed at 7/8 oz. The American 20 gauge was designed to shoot 7/8oz for it's fare but we can buy factory shells up to 1-1/4 oz. A stout heavy gun is needed for such aberrations.
I thnk that, in twelve gauge, the real sweet spot for the 21 is shooting clays. The twelve gauge is very heavy and it is very pleasant to shoot a couple hundred rounds of trap in an afternoon. A 21 has one of the two best and most reliable single triggers in horizon-posed-barreldom. The beavertail forend protects the hand from the heat and it is also very attractive and has lots of shiney surface area on it.
A 12 bore six and a half pound game gun throwing 1-1/8oz is very unpleasant to shoot for more than a box at clays. Certiainly very pleasant for game though.
I mean who doesn't like all those shiney black surfaces? And how could anyhone not like all that shiney mirror finished wood?
Now I have a British sidelock and a Birmy boxlock in twenty gauge, both with 28" barrels and below six pounds, that I really like for Bob White over my pointing dogs. And a sixteen bore, also under six. Really graceful, light, and well built.
But I have a Winchester Model 21 "Trap" grade in very nice condition. 32" barrels with factory ventilated rib, single trigger, beavertail forend, a muscular 8-1/2 pounds, and a non-typical straight grip stock. Oh she's a beauty!
My friend Gary, who is a real 21 officianado, can go on several minutes about the different ways to wax the 21's different surfaces and the advantages and disadvantages of each. Again the Harley analogy.
As Larry said I am sure that someone will chime in here that really likes them.
Best,
Mike