A stationary car bomb has a lot of advantages over other assassination methods. For one it allows you to get away, you only need to plant the thing and it could go off hours, days or weeks later. The assassin could be a in a different country when the bomb goes off. You could put more of them at strategic points into a city not knowing anything about the daily routine of your target, just their license plate would be enough. You could drop a bunch of them ahead of time in random places and program all of them by remote to scan for new plates and so on. Not much you could do about it either, every parked car would be a risk.