(or "The Old Reliable")
Let's start by specifying which "CSI" we're talking about. We're not talking about the CSI TV show that's been going for twelve seasons, nor the other CSI TV show, nor the other other CSI TV show. We're talking about the CSI comics that are based on the CSI TV show that spun off two other CSI TV shows, both of which had impressively long runs as well. So you can already tell that we're talking about a pretty successful storytelling engine here. (What you may not be able to tell, and I'm not going to assume you know, is what "CSI" stands for. It's "Crime Scene Investigations".)
But why? What makes this the kind of storytelling engine that sustains so many stories so easily for so long? Do the comics contain some sort of clue that the TV show doesn't? Actually, it's hard to tell just what the storytelling engine is from one volume of the CSI comic book collections; the "decompression" effect of modern comic books is quite graphically demonstrated (if you'll pardon the pun) with these series. A trade paperback that's over 300 pages long really only contains three individual stories; that's barely enough time to put names to faces, let alone give you a really detailed look into what makes the series tick.
Fortunately, one of the things that the CSI TV series show is that it doesn't really matter who's doing the investigating. All three shows have rotating casts, in addition to having three complete sets of casts, and it doesn't seem to dent the popularity of the shows or the ability of writers to come up with new story ideas. It's the jobs the characters do, not their personal lives, that drive the engine of a police procedural; the occasional episode or storyarc might come from the characters' pasts or personalities, but usually the easiest way to get a police protagonist involved in a story is to call 911. (Depending on the series, of course; 'Criminal Minds', for example, tends to run its one-off stories as basic procedurals and base its longer arcs on the backstories of the people involved. Neither approach is "right" or "wrong", but one definitely lends itself better towards casting changes.)
And likewise, it's not the location that's important; the CSI that the comics are based on happens to be the Las Vegas CSI, but New York and Miami offer no more or fewer storytelling opportunities. Pretty much any big city has a high enough crime rate that you can believably imagine one interesting murder case a week. CSI: International Falls doesn't work quite so well, but you could easily imagine them spinning off the franchise into Dallas, Minneapolis, Los Angeles, or Atlanta without any real difficulty.
So if the people don't matter, and the place doesn't matter...what makes this such a workable storytelling engine? Surely if 100+ previous entries have taught us anything, it's that those things are usually pretty important. Why is CSI evergreen without any of it mattering?
The answer, I think, has to be the crimes. Sad to say, there is an endless variety of human misery out there, ready to inspire us to create art that examines its meaning. Meaning that there's never going to be an end to the inspiration for police procedurals; as long as there's crime today, there will be a story tomorrow that imagines those criminals being brought to justice. That's enough material to supply a hundred police procedurals for a hundred years...which is probably something I shouldn't say too loud. I might give the CSI people some ideas.