The Japanese fanbase isn't the American nH/SS fanbase; who only cared about them ending up together in order to prove they were right then were somewhat content with just fantasizing about the sex. They wanted was a nice romance if anything come out of it. Instead they got: The last, the ending, The gaiden, and Boruto and much more that put the end pairing in a negative light. So if their pairing if it happened was going to be this miserable, why even let it happen?
That is true to an extent. Besides look at it this way. Naruto had a huge Western following. Most western fans were (let's estimate) between 12-23 years old ( of which -not all- are very attracted to shy/busty anime characters). And majority of fans only follow/followed the anime and paid no heed to manga alterations.
Now lets look at the situation: We have Hinata, this busty long haired girl who is shy and always faints before Naruto and is always super nice to him. Don't forget the melons.
On the other hand: We have Sakura, quote "flat" (as they call her), who is always portrayed as a jerk to Naruto and the anime has her punching him to orbit like it's an arcade game like 5x more than the manga.
So logically, most fans began liking Hinata and hating Sakura. Kishimoto took notice of this through letters as he stated in interviews, and began focusing on Hinata more, and shoving Sakura to the side as much as he could. In the start of Shippuden, Kishimoto was making a strong push with Naruto and Sakura which he ultimately shies away from around the time Immortals arc starts when she disappears and the only panel we get is the one where she's in her room crying over the photo frame.
Since Sakura had less and less screentime and was becoming even more distant to Naruto, Hinata was gaining even MORE support. That coupled with her unending addiction to to Sasuke, only adds to the hate meters.
****Big fact: Sakura also never wears her original Kunoichi uniform from early shippuden again, nor does she ever get a big fight/ action sequence again. She's usually portrayed in her village clothes appearing now and then in single panels. Which is a shame because she was the quickest learner, originally proclaimed to be proficient in Genjutsu, she had superior physical strength and on top of that she had incredible healing potential with the Byakugo seal. (Seriously what a waste of a character, Kishimoto)
Now fast forward a bit. During the 4th war garbo arc, Kishi puts all this focus on Sakura. He throws in her comparison to Kushina, she gets numerous panels with Naruto when he's dying and her immense concern on his dreams and hopes. That kinda had me thinking on why he'd go through all that trouble instead of throwing more NH scenes in there to better support the end pairing.
Then I found that interview where Kishimoto stated he spent a large amount of time redrawing & redesigning Sakura, trying to make her look "prettier" and changing her character to be more caring (as is seen in the last 40-50 chapters), in an attempt to alleviate some of the hate off her. But by that point it's too late. You've got over a decade of fans cemented, so it was still strange for him to put effort into something like that. And frankly, if he really planned NH or SS from the beginning, he basically was a master troll for 15 years and 698 chapters. So he really fooled me.
Well, whatever you get my long, rambly point.