For one thing, life imprisonment (LiP) is a pretty big deterrent, and among the few willing to risk it, defense tech and the surveillance state are generally winning the arms race against individual actors and small groups ("terrorists"). Lots of people want to kill world leaders and yet assassination attempts (let alone successes!) are quite rare. This leaves the question of less-defended people, and there it's more mixed: it's much easier to do but 10^6-10^10 fewer people want to kill them which makes it a lot easier to catch the conspirators simply by tracking who had motive. Finally, everyday victims don't usually have haters with enough capital to motivate someone to risk LiP - again, making it easy to track down the perps by motive.