It’s because the left is all about feelings and emotions, and thinks we are the same way. Thus, although they do not care about morality, they stereotype conservatives as caring only about morals. So when they criticize a conservative candidate for supposed immorality, it is a calculated ploy to depress support for that candidate. If enough conservatives can be swayed to sit out the election by the left’s emotional manipulation, then the Democrat wins.
The strategy unfortunately has a high success rate. Voters were persuaded to sit out and not vote for Roy Moore, so a Democrat took what had been a Republican seat. The ploy isn’t as effective against Trump because we’ve known him for decades and it’s hard to spring a surprise on us regarding his behavior. When leftist activists pay women to tell stories about him, we are not very inclined to believe them.