At the risk of sounding tautological, Clinton lost by not winning. The trends aren't in her favor at the moment. Sanders is gaining in Iowa and is starting to be treated like a serious candidate by the press. Her attempts to attack Sanders's support for single-payer health care this week were widely perceived as backfiring, especially among liberal primary voters who might be tempted to support Clinton out of practicality. She needed to a debate in which she could show that Sanders was out of his depth, not someone you could plausibly see actually functioning as president. She didn't do that this time around.
It seems pretty clear that Sanders won this debate. Will it be enough to push him over the top in Iowa?