Scarcity, condition, features, popularity, association.
Only one that really applies is scarcity. It's hardly pristine. Nothing special. Average popularity. Not owned by Elmer Keith or anything. But a 28-1 is very hard to find. That impresses some people. Not me. I wouldn't give more than $400 for it as a shooter. All my guns are shooters. I'm a shooter. Collectors think differently. To them an average condition 28-1 is a 3K gun. One of the many reasons I am not a collector. Way beyond my pay scale. Takes BIG BUCKS to be a serious collector. I'm out. I don't know what these guys do for a living with all the rare, exotic and fancy S&Ws posted on these forums, but I don't do whatever it is they do so I can't afford this stuff. Hell, I can't afford most plain Jane S&Ws at current prices.
The one time in my life I succumbed to the urge to buy a collector gun I got burned. I still like the gun, and I shoot it, but I learned my lesson. I do not have what it takes to be a serious collector. Not the patience, not the savvy, and certainly not the cash.