Reading this Times story on Hillary Clinton's victory in Puerto Rico, I think it's clear why she isn't dropping out of the race:
Mrs. Clinton said from San Juan. “I will have won the most votes — more than anyone in the history of the primary process.”
She added: “Senator Obama has a narrow lead in delegates. And we’re going to have to make our case to the automatic so-called superdelegates. And I think my case is clear — more than 17 million people voted for me.
It's all about your record in politics, specifically the soundbyte on what you've accomplished that you can pin your agenda on. If Hillary loses but comes out with the "most votes in history of the primary" or even a number previously unprecedented (but still 2nd to Obama), that's a tail, even in the context of a loss, she can campaign with in the future. This is her life, so it's worth weathering the storm for preserving.
I've wavered with my feelings on Hillary being so bullheaded, but I think I fall on the side of respect in the end. There's no questioning her spunk and intestinal fortitude. And Obamamania will either sweep the country, or -- regardless of her timing -- we'd lose because of a split party.
Clinton Ponders Continuing [NYT]