Why did Democrats allow Hillary Clinton to coast to the nomination?

I've never understood it. Her only strong competition was a cranky, ill-informed 75-year-old self-proclaimed socialist yelling about revolution. The problem wasn't a lack of available alternatives: liberals pleaded with Elizabeth Warren and Joe Biden to run, but for some reason, they didn't, even knowing they're old enough that this decision might close the door on ever becoming president. And it's hard to believe that Clinton's running mate, Tim Kaine, hadn't considered running for the 2016 nomination after being vetted by Obama in 2008.

In the end, Hillary Clinton proved herself to be a painfully mediocre candidate. It's one thing to lose toss-up states like Florida and Ohio, but the fact that she lost Pennsylvania and Michigan speaks volumes. This happened even though her campaign was far better funded and she was believed to be the only candidate with a serious get-out-the-vote operation.

When you consider how she failed at mastering the delegate game in the 2008 primaries, then went on to win her party's nomination only with bizarrely weak competition (and more narrowly against Bernie Sanders than anyone had excepted), then lost the general election to a historically unqualified candidate, the conclusion is clear: Hillary Clinton is just not a skilled politician. As she openly admitted during the campaign, she lacks the political gifts of Bill Clinton or Barack Obama. It's great that she's self-aware enough to realize that, but why didn't the Democratic Party as a whole realize it before it was too late?