The own Sakura fandom were deeply splited between SS x NS wars at the time with SS fans being extremely vocal against NS. This helped Kishimoto/Editors didn't notice Sakura's popularity especially in Japan. Hinata fanboys still were hating on Sakura in west because she was threat to NH. The manga ended. SS fans got what they always wanted and were free to 100% support Sakura. NS fans left or started to multiship SS. So Sakura fandom grown bigger and stronger as ever...
Most of the Haters simply moved on. Sakura wasn't a threat to NH anymore. No more reasons for Pairing wars. Sakura isn't useless or weak, she isn't punching Naruto every time, has a good daughter, is a good mother etc...
So no surprise why Sakura's popularity boomed after the ending.
Kishimoto should have defined Sakura final pairing at the very beginning: Sasuke or Naruto and never use Sakura to play with love triangles or drama. If Kishimoto/Pierrot didn't trapped Sakura in this ridiculous love triangle with SS x NS x NH or erase Kage Summit arc. She would be much more popular at the time.
Sakura/SS/Sasuke are a controversial characters and Pairing. This helped to maintain Sakura fandom always alive with discussions against haters, theory, fics, fanarts etc. There's nothing to talk or any appeal in Hinata or NH. So this fandom simply died even in the west.
Until a Boruto or Part 3 reboot comes. They simply will produce more novels and spin offs with Sakura/Sasuke to milk their fans. Sasuke always was a popular character and now Sakura is at his popularity level and coincidentally is his wife. So she is the perfect partner for more romances or spin offs. Shoujo mangakas are pratically begging this to Kishimoto...
I wouldn't exactly call the fandom getting bigger if half the fans have left, it just means that the fanbase has shurken so small that ss looks bigger than its use to.
I find her parenting and her daughter to be overrated. Sadara is only like because of one volume from gaiden, she hasn't done much after that execpt in boruto filler from what I have seen. Also it's pretty problematic when sakura couldn't tell her daughter anything about her father to the point she wondered if she sakura was really her mother.
I think it was pretty by the end of part 1 and the beginning of part that narusaku was the endgame, but the fans just didn't want to see it or went all nh.
Controversial characters and pairings won't last long term, you might have an intrest now but that doesn't mean it was last. Look at spider-man, marvel keeps doing one controversial decision after another with his love live and the fans have finally got sick off it at this current run. These sasusaku stories aren't that good and I have seen some sakura and ss fans hate retsuden,
for the way Sakura's character was portrayed. Also naruto is not a shoujo series, making it one will just alienate its shonen base.
They should of just done an open end or introduced the idea of a multiverse. Where ones universe has ns and another has ss. Heck ns could of had the main timeline as that's what the narrate was leading and ss could be in a universe where sasuke was never a jerk to sakura, that should of been what to do if they were so worried about alienating shippers.