Another possible reason is that they want to control the narrative. Fanfiction can sometimes deviate significantly from the original vision of sakura, and they might want to keep a tight grip on how the story and characters are perceived. Also, they may be concerned about potential negative impacts on the brand image if some fanfiction contains inappropriate or controversial content.
One reason could be copyright issues. The creators or owners of sakura might be worried about the unauthorized use of their intellectual property in fanfiction.
It could be part of a new marketing or brand strategy. They might be planning to release their own official spin - offs or expansions and don't want fanfiction to compete or cause confusion. Maybe they also received complaints from other parties, like sponsors or partners, who don't want to be associated with the unregulated nature of fanfiction.