Why was the Book of Genealogies hidden away?

Question

Related.

I read today an essay by Professor Shmuel Krauss z"l about "Sefer Yochasin" (the Book of Genealogies). He theorized that traditions such as the name of David's mother, the identification of Boaz with Ivtzan and many many more such traditions in the Talmud, Midrash and even some Targums may have been part of Sefer Yochasin, which was a baraita to explain and expound upon the genealogies of Divrei Hayamim (per Rashi).

In Pesachim 62b it says:

"Rami bar Rav Yuda said that Rav said the following about it: From the day the Book of Genealogies was hidden and no longer available to the Sages, the strength of the Sages has been weakened, and the light of their eyes has been dimmed, as the book contained the reasons for many Torah laws and lists of genealogies that are now lost."

My question is why was it hidden away? It seems like it contained important information, especially if Krauss's theory is to be accepted - bits of this information were certainly important enough to be included in the gemara! Especially as the gemara explains in Bava Batra 91a that this information was used when arguing with the minim about traditions of Judaism. If it was lost, why not write "מיום שאבד ספר יוחסין" ("from the day that the Book of Genealogies was lost...")?

Answer 1

Rashi to Pesachim 62b explains נגנז as נשתכח. So it sounds like he believes it was lost, rather than deliberately hidden (the question at the end of your final paragraph notwithstanding).

Maharsha ad loc. on the other hand understands that it was deliberate. He links it to the description found in Kiddushin 71a of the Sages ceasing to publicly check into people's lineages, as families whose lineages were declared suspect reacted violently. For the same reason, this list of lineages was hidden away and only taught orally and infrequently, before that practice also eventually stopped.