As I've said before, theories of why Hillary lost will continue to proliferate, and, of course, all are equally unprovable. That said, my current favorite is this article by John Judis on TPM. Some excerpts:
The Electoral College will meet on Monday to declare Donald Trump the winner of the 2016 election. Sometimes, in order to get beyond an awful loss, you have to give up on the rationalizations by which you deny the extent of your defeat.
I have two dueling rationalizations that are prevalent among Democrats: on the one hand, the conviction, based upon Hillary Clinton’s popular vote majority, that she and not Trump was the real winner of the election and that if Democrats can only move the numbers around, they will easily rebound from their defeat in the electoral college; on the other hand, the conviction that Clinton really did lose, but that Bernie Sanders could have won, and that if Democrats follow his example, they’ll regain the White House and Congress.
The Hillary Clinton camp continues to dwell on the fact that she won the popular vote by 2.8 million, even though she lost the electoral college. But Clinton spent twice as much on the election as Trump did, and spent money to drive up the vote in Chicago, New Orleans, and California. According to Politico, the Clinton campaign and the Democratic National Committee were actually worried that while Clinton would win the electoral college, Trump would win the popular vote.
Trump, as his pollster Tony Fabrizio later explained, focused entirely on swing states, and didn’t try to “run up the score” in states like Texas, Georgia and Arizona that Trump expected to win. From October 21 to election day, Trump’s ad spending was entirely focused on swing states, while Clinton was still spending in Texas and California. If the two candidates had spent an equal amount, and if Trump had spent in states like Texas that he assumed he would win and in states like California where his margin was well below Mitt Romney in 2012, I believe the popular vote would have been much closer.
FBI Director James Comey definitely hurt Clinton’s chances when he re-raised the issue of her emails on October 28, but he may not have cost her the election. If you look at the Los Angeles Times tracking poll, which proved to be the most accurate predictor of the results, Clinton had pulled even with Trump soon after the release of the NBC videotape showing Trump bragging about his sexual exploits, but Trump had begun to pull ahead again on October 26, two days before Comey stepped in.
The more significant vote may have been that for House candidates. Nationally, Republicans won 51.4 percent of that vote. By comparison with Trump, the House Republicans did five points better among college-educated whites and one point better among non-college educated whites, and three points better in the suburbs. What these results suggest is that a Republican presidential candidate like, say, John Kasich would have done better among college-educated whites (one of the constituencies that appeared turned off by Trump) and in the suburbs, and held his own among working class whites. If so, such a candidate might have defeated Clinton more decisively than Trump did.
Some Democrats argue that Bernie Sanders would have done better against Trump than Clinton did. I doubt whether that’s true. Clinton’s campaign was abysmal, but Sanders’ proposals for Medicare for all and free tuition at public colleges, which played well in the Democratic primaries (and I supported him and these proposals enthusiastically), would have hit a tax-and-spend brick wall in the general election. Most voters who are not on the liberal/left wing of the Democratic Party will not support anything that calls for higher taxes, even if the proponents argue that in the long run these proposals will save them money. In Colorado this November, voters had to decide whether to back a single-payer system for the state, dubbed ColoradoCare, that was a state version of Sanders’ Medicare for all and that he came to Colorado to campaign for. Worried about the tax bill, leading Democrats as well as Republicans opposed the initiative, and it lost by 79 to 21 percent in a liberal state that went for Hillary Clinton. Trump would have hung Sanders out to dry on proposals calling for higher taxes. Vice President Joe Biden might have beaten Trump by winning Pennsylvania and Ohio, but I doubt Sanders would have stood a chance.
I like the rest of his article too.