So... Why did the Sage leave behind his story only accessible to the Uchiha's?

[Mateush]

Has it been stated in the Manga that the sage wrote the tablet? I can't remember someone please give me the chapter number. If not then I think the Uchiha son wrote it.

I remember wrong about Madara, but Obito said it here:

Spoiler

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Another reason is how the tablet works. Obito said the Rinnegan would reveal more, and as far the Elder Son didn't show Rinnegan. Apparently only RS had it, so it's only logical it was him who wrote the tablet.

 

[Jazzy Stardust]

It was probably stolen sometime before Madara and Hishi's births during that time after ninja started to gain ninjutsu. I always thought it was sketchy that the uchiha had a secret tablet they they read to unlock the sharingan.