What is it about single moms that deems them unworthy for dating?
It depends. I'm focusing my answer on longer term dating and that's limits the analysis. There's a different way to analyze the situations short term casual (and probably non-exclusive) vs. a longer term exclusive committed relationship.
For me, I'm a childless male. I have no interest in raising another man's children with a woman. I don't want to use any of my resources, including my time, on offspring that aren't my own.
Single moms have more complications than childless women. There's a clash in lifestyles too.
I don't tend to evaluate the reasons why a woman is a single mom. I find that a difficult exercise. I acknowledge that there are women who would have somewhat valid reasons for being a single mom. Her husband dying accidently or of a terminal illness would be reasons that wouldn't reflect poorly on her. Leaving a physically/emotionally abusive man is a good idea but likely not going to reflect as well on her because she chose to get into a relationship with him and have kids with him. The same would be true of a woman with an alcoholic or drug addict in her past. More of the failed relationships with kids in the picture are two people without physical abuse, substance abuse, or an unplanned death in the picture. It's two people that drifted apart due to the passage of time. The initial choice to commit to that person is often questionable.
At this point in my time, no one in my dating pool has adult children. Older men tend to deal with women with children 18+. Even that's not a fun thing to deal with for the older man. Adult children often have financial problems and might need to move home at some point as adults. Imagine being a 65 year old guy dating a similarly aged woman and having her 22-30 year old child needing to move home due to layoff or not being able to get a job upon getting a degree. This happens all the time and older daters rarely consider it.
The calculations are a bit different for men with children in dating women with children. Merging two sets of children under 18 isn't pleasant and neither is merging 2 sets of adult children for the 55+ daters.