So the real question is: Why didn't all these Hillary supporters participate in the caucus? The disparity in the caucus and primary results of Washington state is astounding; a 72-28 victory for Sanders to a 54-46 victory for Clinton. That's a swing of 52 points. Nebraska's caucus v. primary results weren't this bad.
Because Clinton basically had the nomination wrapped up at the time of the caucus. Why would her supporters waste a Saturday afternoon at a middle school cafeteria when she's going to win anyway?
With the primary, it's all done by mail, so there's no reason not to vote.