You guys are really asumming tsunade or Sakura doesn't know it .
All of the sanin have probably been to their respective locations
Orochimaru and jiraya know of the senjutsu, it's ludicrous to think tsunade doesn't , especially given that her has the most intimate connection with her summon, they share a mind.
And for those saying Sakura doesn't have the reserves or skill to handle that, Byakugou is a far more advanced form of chakra control, and the power and forcefulness of the chakra when released is basically the same shit just a different toilet. Besides all of the requirements for sage mode is different
But as I was saying nothing suggest tsunade couldn't use it, I'm thinking senjutsu genders the usage of medical jutsu which is why Katsuyu can't heal using her own power. Also tsunade was probably gonna use it against pein , if anything she would have the most powerful sage mode, because it would be instant, flawless, and without limits, its as symble as reverseing the Byakugou connection ( which she did in order to see via Katsuyu)
So the true answer , like anything concerning Sakuras development is that it was too risky for the plot, Sakura would potentially be too powerful
( think about it , she is already virtually unkilabke (beheading is fanfic,, this isn't marvel) and if you ad slug like ability to her arsenal ( given that Katsuyu is unkillable) + her chakra enhancing and over all talent in basically every natural ninja skill , she would be too perfect, and outshine her team mates once again.
