You'd sort of have to ask Caesar because none of our texts mention any motivation. A better question, though, might be why Caesar would have agreed to divorce Cornelia. From the time of his father's death Caesar was a partisan of the Marians and was already by 82 an enemy of Sulla's. Divorcing Cornelia would mean resigning as flamen dialis, an office that required marriage, and admitting his loss to Sulla, neither of which can be expected to have been acceptable options to Caesar the young partisan--when during his life was Caesar ever willing to back down when challenged by a political enemy? No matter what attachments he may have had to Cornelia (and we simply have no idea) divorcing her was acceptance of Sulla's indirect demand that Caesar be silenced. Even after being deprived of his office as flamen dialis and of Cornelia's dowry Caesar obstinately refused to divorce his wife--there was more to his marriage to Cornelia than that. Caesar was, from the very start, in a dangerous political position, and why should we expect Caesar to uncharacteristically bend to an act of political terrorism?