Because naruto fans are stupid. When the manga ended everyone complained about how kaguya was an underdeveloped villian(rightfully so) now that we are getting some backstory in the anime, people are whining because its the anime so it isnt technically canon.
The problem is that people are too hung up on what is canon and what isnt. If its good who gives a shit? They are getting the content they wanted, but in anime form, so its a problem. Just enjoy it for what it is instead of worrying about whether it should be considered a full fledged part of the story. If its good consider it as such. Its all fiction to begin with anyway....unless these fillers seriously contradict something in the manga, stfu and quit crying like a no life virgin weeaboo.