Because it “was her turn”. LOL.
And also, the bench of potential Democrat candidates was unusually shallow and thin. After some 24 years of the Clintons breathing up all the oxygen in the room, except for a short period in 2008 when George Soros switched his support to a relative unknown and snatched the nomination away from Herself, then giving Herself a consolation prize of Secretary of State for four years. There was little if any chance for ANY development of a potential successor. Not in the Senate, not in the governorships of the various states, not in the House of Representatives, not even in the various appointive offices.
A presidential candidate nominated just because it is his/her turn is sure to lose. The Republicans should have learned that with Bob Dole and John McCain.