To put it simply, because she cannot win. And by staying in the race, she is only hurting the chances of Democrats winning in November. Politico analyzed the math and wrote a story on Friday titled "Story behind the story: The Clinton Myth."
Here are the highlights (emphasis mine):
One big fact has
largely been lost in the recent coverage of the Democratic presidential
race: Hillary Rodham Clinton has virtually no chance of winning.
Her own campaign acknowledges there is no way that she will finish
ahead in pledged delegates. That means the only way she wins is if
Democratic superdelegates are ready to risk a backlash of historic
proportions from the party’s most reliable constituency.
Unless Clinton is able to at least win the primary popular vote — which
also would take nothing less than an electoral miracle — and use that
achievement to pressure superdelegates, she has only one scenario for
victory. An African-American opponent and his backers would be told
that, even though he won the contest with voters, the prize is going to
someone else.
People who think that scenario is even remotely likely are living on another planet.
As it happens, many people inside Clinton’s campaign live right here on
Earth. One important Clinton adviser estimated to Politico privately
that she has no more than a 10 percent chance of winning her race
against Barack Obama, an appraisal that was echoed by other operatives.
In other words: The notion of the Democratic contest being a dramatic cliffhanger is a game of make-believe.