She is the only candidate who has seen her partner do the job, she witnessed from the front row one of the best presidencies in modern times. She was alleged to have been more actively involved than any other first lady ever. (Something many Conservatives criticized her for in the past.) She spearheaded and lobbied for the last attempt at healthcare reform. She has helped run two major presidential campaigns (while not necessarily a qualification to be president, one for the Democrats to consider).
Honestly, are there any Republicans even close to being as qualified?
Were Nixon, Ford, Carter, Reagan, Bush Sr., B. Clinton, GWB, or Obama as qualified?