Spider-Man is a regular syndicated strip, not appreciably different than Garfield from a distribution perspective. So, while it's perhaps more typical for a paper to run a number of strips from a single syndicate, there's nothing stopping a smaller outlet to get a single strip and run that.
Spider-Man makes sense for Comic Shop News. It's designed specifically to be distributed through comic book shops, and tends to focus particuarly on items of interest for a regular comic shop visitor. What is sometimes (somewhat derisively) called The Wednesday Crowd, who tend to have a greater interest in the superhero and related sci-fi/fantasy genres. CSN could probably also have tapped The Phantom, Mandrake the Magician or Flash Gordon to run just as well.
The question one might ask is: why would CSN run any of those strips at all? Aren't they all way past their prime? What benefit would readers have in that content taking up space in paper's pages? Well, the same could be said of newspaper comics more generally, right? It's the same arguement. It might not be the primary reason people pick up the periodical, but the more reasons you give them, the less likely they are to drop it. The strip's content adds a layer of material that's distinct and different from the typical news, makes for a small delighter to the consumer ("I can still read about Spider-Man without having to buy the comic book!") and, from a publishing perspective, it makes it easier to fill out the pages on an ongoing basis.
There are any number of comic strips out there, of course. The classics that you're possibly more familiar with, like Beetle Bailey and Garfield, but many take on genre and stylistic trappings that cater to specific audiences. Dilbert, for example, speaks primarily to life in an office environment. Retail speaks primarily to life in a, well, retail environment. Frazz takes place in a grade school, with frequent commentary on running/biking/swimming. Every comic strip has a hook that helps to make it stand out, and that same hook can be used as an identifier of a niche audience who may be more receptive to its style.
Where I'm going with this is that comic syndicates' traditional market -- newspapers -- have been on the decline for several years and things look increasingly bleak for them. Which doesn't bode well for syndicates either. So what if syndicates changed their direction a little to focus, instead of on the broad mass media newspaper outlets, on the any number of smaller outlets that might only be able to afford a single strip? What if they took those niche comics and sent them only to outlets that might cater to those same niches? What if Retail ran in Kohl's internal newsletter? What if Mutts ran at the bottom of an animal shelter's emails?
More effort on the syndicate's part? Sure. But with newspapers both dropping as a whole, and cutting back on what they do carry, it seems to me that syndicates must be facing down some really tough challenges right now. I'm not "in" well enough with anyone at the syndicates to know how things are really going and what they might be working on already, but from my armchair quarterback position, I'm not seeing much of anything beyond more of the same and that sure isn't going to be sustainable.