Statistically, the groups that make up the lefts' base (young people, african americans, single women) are simply less likely to vote. In some circumstances they can be pulled to (for example, a candidate they are excited about), but generally:
They're more likely to not bother.
Sort of - you have it backwards. Romney was to the GOP in 2012 was Hillary will likely be to the DNC in 2016 - a candidate that doesn't really match the base, and thus depresses turnout...you have a strong right-wing base to the Republican party that uses its threats to get the candidates to cowtow to their issues. It is probably why Romney lost. If he hadn't sold his soul to the right-wing and flip flopped on every major issue and stayed true to his convictions....but instead he felt the need to pander to the right-wing base and he couldn't shake his etch a sketch enough to reinvent himself for the GE
Americans have gotten more polarized over the past few years. A winning national-level candidate now is one that excites their base without scaring off the middle.