Both are unlikely places for an assassin to strike, largely because of how rigorous the Secret Service checks and security were in these two locations. Both were locked down and the Secret Service, as we know from documented evidence, were running a tight ship at both in order to prevent assassination. Not even a 'we've done enough so it won't happen, lets be a bit more lax' attitude was taken, they were genuinely anxious someone would take a shot at the President and were thoroughly prepared. So they had complete control and command over everything going on in those locations, and had a threat arised, would have been able to quash it long before it reached the President.
Which is why that in the only place they didn't have complete control over the situation, the President was shot.
But lets assume someone makes a mistake, and someone who wants to kill the President gets through either with a gun. Pretty big if's, but hey, lets assume that the SS miss a spot. Kennedy being shot in front of the nation is going to be more traumatic on the national psyche, even if he survives. Not only will he have been shot, but millions will have witnessed it 'first' hand.
