Peggy Noonan explains:
A largely unremarked fact of modern presidential politics is the increased and wholly understandable reluctance of candidates' families to agree to a run. Looking at it through a purely personal prism, and that's where most people start, they see it not as a sacrifice, which it is, but a burden, a life-distorter, and it is those things too. But they have to agree to enter Big History, or a candidate can't go. And a lot of them don't want the job, if victory follows candidacy, of "the president's family." The stakes are too high, the era too dramatic, the life too intense. They don't want the intrusion, the end of all privacy, the fact that you're always on, always representing.