Because the vast majority of Christmas consoles that are purchased are for children and we are now reaching the lower price points that will make that an even bigger pull for more and more buyers. Neo won't occupy that space, it will be an ethusiasts machine or machine for the hardcore.
Don't mistake what I am saying for any kind of suggestion that the Neo will fail or fail to seek an audience, however it will not outsell the normal PS4 anytime soon.
At $399 it probably will if they do a good job playing up how the additional power impacts games.