- They're inexpensive. Old cop guns and "Value Series" 910s and 410s are a glut on the market right now. It's a cheap branch of collecting to get into, kind of like S&W revolvers used to be.
- There are a bewildering array of variants of varying degrees of scarcity.
- There are all manner of police department-marked guns out there, and collectors dig those, because it adds yet another layer of things to collect.
- There's room for a lot of original research out there. Twenty years ago, nobody in this country gave a rat's ass about, say, Finnish Mosins; now you can practically get an online degree in the field.
- The classic stainless S&W auto appears in lots of '80s/'90s iconic movies and TV shows, and that's the next big wave of retro.