I assume it is because more times than not the nomination is clinched earlier than June. She was looking for examples that fit her argument. Everyone does that. Watch the video and see what a big nothing her comment was in context.
Right. She was looking for examples and had to go back forty years to find one. Which makes the example pretty irrelevant.
I did watch the video. She should have said Bobby was running in June, not assassinated in June. Perhaps someone of decent character could get away with saying it, but the Clintons' deserve every suspicion they get.