It doesn't surprise me that Donald Trump would claim that Ted Cruz's father was seen with Lee Harvey Oswald shortly before JFK's assassination.
Honestly, I'm only surprised that Trump didn't claim that Rafael Cruz wasn't part of John Wilkes Booth's conspiracy to assassinate President Lincoln.
Why does Trump do such things? For the same reason that he says he could commit mass murder in Times Square and not affect his poll numbers. Because he can.
These are the excretions of someone who lives in the gutter. But because Trump lives in any palace of choosing and because he's been on TV for so long, a critical mass of this country accepts what he says at face value no matter how stupid it is.