Planar NAND was scaled and at the end limited by the cost of lithography, wheras 3D NAND scaling is enabled by advanced deposition and etch processes defining complex high aspect ratio 3D structures. Here is an excellent article by Lam Research in Solid State Technology on the challenges in 3D-NAND fabrication.
Solis State Technology : LINK
Screen capture from Solid State Technology online magazine (LINK)