It took me forever to think about this, but it seems like any time the spouse isn't the killer, an inordinate number of victims - at least in the original series - are unmarried. Or at least they're unmarried as far as you know, and a lot of them for certain. (Even if some of them are "between marriages," like Ric Carsini.) I can only think of about ten in the original series who definitely are married, which isn't a tiny number for the number of episodes there are altogether, but it's still pretty small. (And in at least one of those, BY DAWN'S EARLY LIGHT, you never see the spouse and you only hear one reference to her.) It's not that I have one myself, but are there any ideas about why that is?

It could be that it might be more upsetting to TV viewers if a married person gets offed rather than a single person, especially if there are children involved. Remember it was 1970's tv and back then even the more violent shows would not venture into the realm of murdering a person who had children. A good example is the movie "Brian's Song" At the end of the movie they mentioned his three young children that he left behind after Brian Piccolo's passing, yet they never showed the children in the movie. It would probably be too upsetting for tv viewers.

By not giving the victims a spouse, children, a detailed backstory, etc., it also makes the victims less sympathetic which could also be the part of the goal. Part of the open mystery format is to give the viewer the opportunity to see things from the murderer’s perspective and sympathize, up to a point, with the murderer. Feeling too sorry for the victim might undermine that.

Or it could just be that the writers worked so hard to concoct the elaborate murder plots, they didn’t have much energy left to put into writing for the victims. (I also think the murder motives weren’t always given much development, either.) Plus, they didn’t have to hire actors to play victims’ relatives.

Yes, that all makes sense.
It's clever that the show would sometimes use them as very temporary suspects. One very good one is Ruth Stafford, who doesn't mind telling Columbo that she has what would be called a motive.

I don't think Ruth Stafford really wanted the divorce.