John Cole gets it, I think:

There are several things at play here. First, the reality that Clinton is not going to win the nomination. Second, the reality that this signals the end of Clinton dominance of the Democratic party (though anyone who thinks they are now irrelevant is both wrong and stupid). Third, the notion that the Clintons do not want to cede control. Fourth, the fact that the only real political power the Clinton family has left (Bill remains a popular ex-President, Hillary is a junior senator from New York) is the support of their voting bloc, who have remained quite loyal to them. Finally, the desire to do as well as they can in the elections today to shore up their last bargaining chip. …

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In short, the message being sent is that she knows she has lost the nomination, for now, but they are not defeated, and she will drop out once she has what she wants.