There's a history
I disagree with John Aravosis' setiment that Clinton was invoking Bobby Kennedy's assassination as a reason to stay in. But he points out that Clinton made a similar comment back in March:
TIME: Can you envision a point at which--if the race stays this close--Democratic Party elders would step in and say, "This is now hurting the party and whoever will be the nominee in the fall"? CLINTON: No, I really can't. I think people have short memories. Primary contests used to last a lot longer. We all remember the great tragedy of Bobby Kennedy being assassinated in June in L.A. My husband didn't wrap up the nomination in 1992 until June. Having a primary contest go through June is nothing particularly unusual.What this demonstrates is that this latest incident is not a simple case of temporary in artfulness. Instead it is a case of several months of cluelessness about just how sensitive people are about this topic.