Well, geometry, I would imagine. In a right triangle with the two sides adjacent to the right angle being equal, the hypotenuse (the long side) is 1.414 times the length on one of the shorter sides. So 1.5 times the depth gives you basically the depth of the water for your indicator to be other than directly above the fly, while still keeping the fly on the bottom. In other words, if the water is 5' deep, and your indicator height is 7.5', your indicator can be up to 5' from being directly over the fly, and there is still enough leader for your fly to get to the bottom.
DG
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