The great assumption was that the strategies were uncorrelated, and would not all move in the same direction at once. If they did it was called an anomaly and discarded as an unreliable data point. Its remarkable how many investment advisors still subscribe to this. In fact, investment advisors who don't are regarded as "the Fringe".
Today managing the edges is not enough. There are no edges to manage, and its time to reweave the carpet.