Trying to find “the reason” is a fool’s errand. Tens of millions of people voted for Trump, and each of them did so for different reasons. You can find any set of people who total more than the margin in the key states, and say that whatever their reason was is the reason that Trump won. Or you can find some completely different set, with a different reason, and say that’s why. Or, heck, most of those people probably had multiple reasons for their vote, and would have voted differently if some number of those reasons (but not necessarily all of them) were different.
And, of course, “why did we lose before?” is not the same question as “how do we keep from losing again?”. The answers to the former may offer insight on the latter, but sometimes it’s insight that we don’t want or need. For instance, some Trump voters undoubtedly did so because they knew that he was a racist sexist bigoted asshole, and that’s exactly what they wanted. We might be able to win those people over by running a racist sexist bigoted asshole of our own, but we’re not going to, because the whole point is to get rid of a racist sexist bigoted asshole.