1. His pedigree. Yes, it's unlikely, but the Kennedys seem to attract tragic endings.
2. One assassination attempt already. I'm honestly worried about copycats at this stage. Same goes for the rest of the candidates.
3. Goodwill. I don't see Kennedy attracting all that many votes from Biden, but with the announcement of Vance as Trump's VP, Trump has also basically made it clear he has no intention of moving to the left, and this is going to alienate a fair amount of soft Trump support him on the off-chance that he'll "reform".
4. Excuse to reassign Trump's SS detail. This one's a bit convoluted, but I can't help but think that it was awfully convenient that a 20 year old young man with no military training was able to elude crack SS guards, climb one of the only unguarded buildings with a line of site in the area (take a look at the fairground map), set up and shoot several shots - and that he was mysteriously killed before he could be captured. By beefing up security, Biden can also clear out those who might be more loyal to Trump than they should be.