Because she loved him. She really did and that shocked people.

1) "During her widowhood, Jacqueline was romantically linked by the press to a few men, notably David Ormsby-Gore and Roswell Gilpatric, but nothing came out of it. So when the news of her marriage to Aristotle Onassis broke out, it came as a total shock to the world. Her motives for the marriage are open for debate, but beyond financial security, it is reasonable to believe that at that point in her life she desperately needed an escape from the Kennedys and the United States, as she came to fear for her life and that of her children after the assassination of her brother-in-law Robert F. Kennedy in June 1968. The wedding took place on October 20, 1968, on Skorpios, Onassis's private island in the Ionian Sea, Greece. Jacqueline gave up Secret Service protection and her Franking Privilege, to which a widow of a president of the United States is entitled, after her marriage to Onassis. For a time, the marriage brought her much adverse publicity and seemed to tarnish the image of the grieving presidential widow, and she became the target of paparazzi who were following her everywhere much to her displeasure and dismay. Despite it all, the marriage initially seemed successful enough, the couple dividing their time between New York City, Paris and Skorpios. Then tragedy struck again, Onassis's only son Alexander died in a plane crash in January 1973. The once invincible Onassis was left a broken and disillusioned man and the marriage turned sour. His health began deteriorating rapidly and only his death in Paris, on March 15, 1975, saved Jacqueline from the embarrassment of a divorce. Her legacy was severely limited under Greek law, which limited how much a non-Greek surviving spouse could inherit. After two years of legal battle, Jacqueline eventually accepted from Christina Onassis, Onassis's daughter and sole heir, a settlement of $26,000,000, waiving all other claims to the Onassis estate." Source and further information: http://en.wikipedia.org/wiki/Jacqueline_Kennedy_Onassis 2) "Onassis ended his relationship with Callas to marry Jacqueline Kennedy, widow of U.S. President John F. Kennedy, on October 20, 1968. It was said that Kennedy insisted on marriage rather than an affair so as to avoid upsetting her children." Source and further information: http://en.wikipedia.org/wiki/Aristotle_Onassis

because he had a really cool name.