She was already preferred by half of the party in 2008. The other half now likes her for being a good SoS and a team player. She's a woman in a party that thinks it's long past due that a woman should be president. She's the wife of an extremely popular former president. She was widely approved of and praised for her tenure as SoS, even by Republicans before they got terrified of her running again and drummed up "Benghazi!!1111!!11!!1". Aside from her hawkish streak on foreign policy, her policy views fit the party as a whole. She's one of the most experienced candidates to run in decades. She also polls far better than everyone else, which makes people want her for electability reasons as well.
This. I would add that the primaries made her the most well known contender for the Presidential nomination - and when you add everything else IceSpear said, the numbers make sense.
