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.