A question that is related can be asked:
In the past, how come the true revelations were not maintained in pure form by believers? How come that which prevailed were fabrications?
And it's not a hard or difficult question, I simply want you to realize Quran has all the questions people pose regarding guidance and it's nature and the various ways humans are allowed to react to it but that guidance still remains in a clear fashion.
In this case was the Quran unclear on how falsehood prevails in the mind of the masses in the past?
It gave warnings to avoid and clear signs towards the way but those don't avail to a people who don't see with the eyes of love and hence do not believe.
Now I believe the truth has been conveyed over all in our ahadith, but us not getting exactly everyone who is trustworthy to be really trustworthy and not everyone who is deemed liar to truly be liar is expected given the oppressive conditions we found ourselves in.
The reminder remains, the message remains, the Quran calling back to the leader of our time remains.
If you want to know why books disappear, you have to understand what Quran has exposed of the deceivers among men and Jinn, and how they work behind the scenes.
Books don't simply disappear, the hadiths don't simply disappear, they purposely hidden away and there is much going on that people are too cowardly or ashamed to talk about.
The people who followed the sorcery attributed to Sulaiman never disappeared, and remain hidden, all over the world, and are doing their best to bring about their world order.
Satan and his forces have deceived them, and brought them under their fold.
You cannot and will never be able to prove your scholarship by means they prevailed, because Quran rejects such thinking and so does reason.
In fact the world being a mess, the umma being a mess, the ones who prevailed are obviously not the guided people.