RFK has done enough things in his own life to badly besmirch himself. The book probably does come out now because he’s brought the Kennedy name back into the news, and the author wants to capitalize on that, but I doubt that ruining young Bobby’s campaign was the author’s main motivation.
The author probably wrote most of this book a long while ago. And the publisher decided to promote it now. The author is happy that a Kennedy is in the news for book sales. The publishing company is a propaganda company, as most American publishing companies are. So, they put some money behind this book because it nudges the country away from idolizing Kennedys.