I think they don't really want the game to be down to quickscoping. If the Sniper would kill in one, why would anybody use anything else than a Sniper (Faster to kill than with any other weapon on practically any range, just a bit tricky sometimes)?
Making it only kill on headshots forces you to aim with your quickscopes. It's still probably the most efficient way time-wise (because you can reaction-fire and still kill somebody) but it's really hard to do.
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