Kennedy had a strong youth support base that, arguably, were much more leftist and radical than even Kennedy would have been (how quickly everyone forgets the escalation of Vietnam, Bay of Pigs and Cuban Missile Crisis).
He was killed early, so people get to take those radical and unrealistic expectations and turn them into truths. He is remembered for the future people think he would have brought, and for the whole spectacle thing.
Actually, come to think of it, there might be a lot of similarities with Trudeau, though who cares about Canadian politics 😉