What a great question. Bio weapons lack the visual signature of nukes, lack that visceral reaction one gets seeing the small shacks and plastic dummies getting destroyed in black and white, don't leave the charred outlines of their victims on the sides of stone buildings for all to see for years to come. They're sneakier and easier to ignore by the masses. On the other hand, they have persistent effects and area effects even greater than nukes in the long run. Seem more user friendly. Probably would have solved our global overpopulation problem by this point, and with that, a host of other problems as well. Maybe not the way you'd want to solve a problem, but, if you want to make an omelet...