Steve Benen considers lists of reasons offered by sources like the Wall Street Joural and Time as to why the once seemingly-inevitable nominee ended up disappointing.
From "misjudging the mood" to "Clinton fatigue," he sums them up nicely:
She went positive when voters were less engaged, and negative when voters were more engaged, which became a drag on her favorability numbers. The media that was too quick to call her the presumptive nominee became something of a foil. The “sniper” controversy and the “gas-tax holiday” idea raised serious doubts about her credibility.