This is because she seems to be incapable of realizing that her assumption that if she catches a man doing something, he'll be so embarrassed, he'll HAVE to do right by his wife and never stray again is, well, stupid and wrong. All parading down Main Street bellowing about how yessir, she'd put a stop to something would have done is to make her look like a nosy fool who made the friend she thought she was helping feel betrayed and hurt. What's more, she'd take out her anger not on the husband who did it but on the meddling imbecile who witlessly destroyed a cozy little illusion.
She also doesn't seem to want to accept the fact that said exposed husband would simply have to find a better way to hide the impulse towards polygamy that drives him; Annie's casual revelation that she'd pretty much turned herself into Batman in order to keep Steve from straying might have surprised Elly but it doesn't shock me overly much. This is because I remember all the strips that had Ted get either dismissive or defensive when John told him that he was headed for a fall or become The Amazing Buckpasser when he did crash and burn.
Finally, she seems to be burdened with the insane assumption that impotence means assent. As I've said before, she thinks that a whole town loves adultery because they're paralyzed into inaction by a situation that they don't like but can't do anything about without making things worse. This last peculiarity seems to be why she has such a hard time learning from her mistakes. Simply put, she thinks that she matters more than she actually does and it tends to screw up her life a whole lot.