Because it's a hard task to animate giant full hd shit nowadays. You can't just draw like the past on a list of a4 paper and scan it and boom, you now have to draw digitally in 1080p or even bigger resolutions, so you're essentially drawing on a bigger canvas, because it needs more detail to look good, not simple/stretched. And it's super expensive and time-consuming to do 24fps animation by hand and pretty hard, you need very very good animators, not just artists. With keyframing and mocap even beginners can do amazing shit.
For humanoid proportion like all jap animation is, realistic looks better than hand-animated, so naturally mocap/rotoscopy with high frame rates - 30/48/60 should be preferable.
What they tried to do, I assume is to make it feel hand-drawn right from fucking 3d application, which looks bad.
Also digital!=3d. The 2d industry has been digital for years now.