Same reason a kb/mouse is better.
Dual Analog sucks IMO. I've take far longer to get used to it and become a halfway decent player than I did when I played my first FPS on PC or Wii.
DA requires a lot more precise thumb control and is not a natural way to point and shoot something. If Battlefield 3 utilized Move, I'd buy a Move set immediately.
Its just faster to point and shoot and you can very easily shoot in a single spot. In Conduit and the COD I played on my Wii I was constantly getting head shots and quick kills. In BF3 I find myself shooting mid-body most of the time and having to pull trigger longer. Sure there are differences in the game dynamics, but the aiming is far harder with a thumb stick... and slower.