...because in my experience it handles moisture (rain, sweat, and condensation) better.
I don't actually walk in it much, and when I do it's usually under a rain shell. In that case it seems like the inside of my shell might wet out a little, but the nano doesn't get clammy like down, and doesn't lose it's insulation. Once in a while I'll use just the nano for lunch, resting, around camp, and above timber-line; in that case I appreciate not having to be as careful about keeping it dry as with down.
True, down is a better insulator, pound-for-pound and maybe even by volume... In the store. But for stomping around outdoors I'm not sure that's true, and in any event both jackets are so light and compressible that for my purposes the trade-off favors the nano.
Just my 2¢ worth...