I've wondered whether there are rules to the game that prevent certain pieces from being just 'taken out'.
In the olden days, charisma or force of personality was enough to prevent a peasant from killing a king. If the king was dressed in sackcloth without any of the regal trappings (i.e. items of power) it might be a different story. Missiles though, are a different proposition.
One possibility is that Soros simply isn't meant to die yet, and killing him before the lines of fate converge is an act of evil that somehow empowers the enemy. It may also altar the timeline in such a way as to prevent to future we're aiming for.