Despite the occasional perspective-skewed take from the far right, the mainstream Democratic Party, as represented/controlled by the Democratic National Committee, is a barely left-of-center corporatist organization. Actual leftists are deeply disappointed and distrustful of the party leadership, and I'd venture that the majority of people who would describe themselves as "leftist" or "progressive" are not registered Democrats. After all, the largest single group of registered voters are independents, and have been for a while. Many of those voters often choose Democrats out of a sense of obligation (or simply as a vote against the GOP candidate), but with little enthusiasm, and turnout is often poor. That can be a problem with the GOP, but the whole Trump phenomenon is, to be blunt, cultish af...and his core voter has been reliable about actually turning out.

A lot of further-left voters have never forgiven the DNC for their thumb on the scales for Clinton in 2016. Many of those same voters will have thrown up their hands over how the Party handled Biden's decline and the no-vote nomination of Harris. The perception of the further-left of the Democratic Party simply being a corporatist party that puts on a show of actual progressivism with little actual progressive substance is strong (and justified).

The Democrats suck at presidential campaigns. Or at least they have in recent elections.

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The issue of transgender women in women's sports was indeed a bigger problem for Harris than I think many on the Left realized it was going to be. A pretty big (but not majority) contingent probably never thought their position that this was okay was actually quite unpopular with a large faction on the left, to say nothing of more centrist swing voters. Giving women and girls a chance to have their own sports categories to fairly compete in was literally the work of generations, and we (yes, I'm one of those leftists on the scientifically sane side of that issue) deeply resented the work of trans activists to undo that progress by insisting athletes benefitting from the enormous advantages having undergone male puberty confers be allowed to compete against those who have not. Gender (as a social phenomenon) is irrelevant to athletic competition; biological sex is what matters in this scenario.

Many, many leftists (along with moderates and conservatives) rejected that misogynist nonsense. There should be no doubt this weakened Harris' campaign. Probably more a matter of demotivating voters than outright turning them to the other side (excepting some of the more right-leaning undecided moderates), but suppressing Democrat turnout had a big effect.