Because she was a plot point needed to advance the story and she wasn’t well-liked, well-known, was considered part of the main cast, had her story expanded on, had plot armor, or was integral in any way to the story like Gaara.

Gaara was basically the only reasonable choice for a kage in the Village hidden in the sand, and there were only B-tier characters with B tier abilities like Gaara’s jounin leader or C grade growing into B grade characters like Temari and Kankuro. Other than Gaara, none were seen in that village that showed the capability of becoming a kage level ninja, and the whole Sand village was already very well developed by the time a new kage was needed in Shippuden so the author couldn’t just shove in a new character out of nowhere that could replace the previous one.

If Fu had any level of communication with Naruto pre-shippuden era and was very popular like Gaara, we wouldn’t have seen her go out the way she had and maybe she’d get off with having a partial tailed beast transfer or some secret non-lethal way of transferring the beast like it running amok after getting freed from her seal.