Because Studio Pierrot are a bunch of asshat corporate pricks who would never waste their precious money on decent animation so long as the series still has high ratings. They usually only put their highest budget in main or ending arc fights since they probably calculate that's when they'll get the most ratings and those are all that matter, which was pretty much the case for most of the war arc. The animation had poor detail and a lack of frames until they got to the edo kages and Madara.
I'm honestly surprised with all the crap they've pulled that Kishimoto has stuck with them and not switched to another studio. Maybe he just can't. Maybe he's under contract, maybe no one else will do it (although I would find that hard to believe), maybe he likes the animation, though from what I hear, the studio has pissed him off a few times with their adaptations. For example, they made Kurama breath fire in the Pein arc and I read he was outraged by it. Or maybe it's essentially the same. So long as the anime gets good ratings he'll stick with the studio.
