Possibly because it has been in use (the technique, not the product) for years. When I was in the Corps of Engineers in the late 60's we were using it. The WHO and the Wilderness Medical Society have been suggesting it as a possibility for years (I don't know how many). I have used alum for reducing turbidity in nasty places for a long time, always as a precursor to other techniques. Used in dirty water before filtering it makes the filter last LOTS longer. Also makes the chlorine work better. Use a normal amount, not several times that much. Very obvious if you use the detection strips from a MIOX system.

Respectfully,

Jerry