She is not popular. It may be that she is clever and competent, but if people outside the inner folds of the Democrat Party don't warm to her, she will never get the mass of the American population to vote for her in the numbers required to win. It isn't the 'true believer Democrats' you need to convince, it is the mass of the population at large.
Too many politicians the world over forget this. It is why Labour lost so badly in 2019. outside of the party fold, and outside London, ordinary people up and down the country could not stomach Corbyn.
Sunac is another example - quite apart from the fact that he had a huge uphill task because of the previous incompetence of Boris ans Truss (a massive task) the fact that Sunac is clever and thoughtful, and came originally from an ordinary middle class family, were nowhere near enough to cut it. People just didn't like him enough. He did not look like a man you would follow across nomans land. You MUST have a candidate with big appeal, not just brains and policies, if you want to win. Harris has no appeal and will not win.
I despise Trump for reasons I have often expressed, but I think he will inevitably win.