Hillary lost because of the Comey memo about reopening her case in the last days of the campaign and how the Trump campaign messaged that to people.

Every presidential campaign since Reagan has come down to the state the race is in during the very last days. Turnout is everything, and it's why Trump could very easily win this.

The point is, the dems won't change their strategy because they don't have to. It has a certain probability of working and isn't predestined to lose.