A variety of reasons but I think the most salient thing I read is about gender and that women perceive voting for a man as similar to voting for their father or their husband. With that in mind, you need a particularly compelling woman candidate to overcome the sort of ingrained expectation that your dad should be making these decisions, not your mom. And, yes, that sounds sexist as all hell but it's how many people, women included, think.
One of the primary reasons that I always thought Trump was being underrated as a 2016 candidate (and I think I wrote about it on here back then but I'm not sure) is that he had a longstanding reputation as a success figure. As a brand, "Trump" was extremely strong and his personal failings had little to do with his branding as a successful business magnate. Years after the fact, I think any rational person should understand just how much of that brand is smoke and mirrors but we also know that it's hard to change a brand's image. Which is why it's foolish to assume that he will lose 2020 as a result of his current candidacy - he's had failings but nothing so significant that it changes his ingrained brand for people who formed their sense of the world in the 1980s and 1990s.
So, I expect that we're going into 2020 with a scenario that's pretty similar to 2016. And in 2016, women didn't dump Trump over his behaviors and I don't think they will now. That means you need fence sitting women to vote for a relatively low experience woman over a man with a term's worth of experience. I don't think it happens.
