Because a non-trivial outcome of Trump's assassination would have been the immediate takeover of the presidential candidacy by a member of the remaining Neocon cult. I hope no one even doubts that. If they are unable to get rid of Biden due to his current bout of dementia-driven stubbornness (plus his natural-born arrogance), the second best option is to have their own candidate on the other side too, as they had always had until Trump broke the rules.