One reason could be that historic angle. In 2008, Clinton didn’t make as much of her role as a woman as she might have. In part, that’s because the “historic candidacy” argument didn’t really cut in her favor in a race against Barack Obama. But Clinton set the tone for her 2016 run when she bowed out of the 2008 race, when she said, “Although we weren’t able to shatter that highest, hardest glass ceiling this time, thanks to you, it’s got about 18 million cracks in it.” This time around, she’s been more willing to put gender front and center—speaking repeatedly about her roles as a mother and grandmother, for example.
But Clinton’s ability to capitalize on that is somewhat undercut by her own unpopularity. Voters might be interested in the idea of a woman president, but they’re not necessarily excited about the idea of this particular woman president. Adding another woman to the ticket might help encourage the urge to vote for an historic ticket. Emphasizing the woman angle might be especially effective against Donald Trump, should he win the nomination, given his long history of demeaning comments (and allegations of worse) against women.
On the other hand, Trump already polls extremely poorly among women—in a recent Gallup poll, 70 percent had an unfavorable impression of him. Meanwhile, Clinton beats him handily among women in head-to-head general-election polling. (She also beats Ted Cruz by a healthy but slimmer margin.)