Because I don't know of any, they manage to destroy a text here or there, but there were always some compatible texts that survived. AFAIK, we really didn't lose useful knowledge to them.
(And let's not forget that today we made life much harder for the book burners.)
What really does destroy knowledge is it becoming useless for a long period of time. As in losing all of CS because nobody can make a computer yet. But we have an instinct to preserve knowledge, even when it's absolutely useless. This has already saved a lot of technology, and it's hard to imagine how some disaster would stop it.
Personally, I have a hard time imagining something that would throw us at the stone (or bronze, or iron) age, but still fail to destroy all the life on Earth.
Digital information seems especially likely to me to be lost over time because the recording nearly always requires an instrument to extract and these instruments do not have nearly the durability of books.
If you want to talk about early Christian texts, the Didache is instructive. It's orthodox and never was suppressed. It's earlier than much—if not all—of the canonical Greek scriptures. However, the canonical scriptures overshadowed it and it became obscure, to the point where the only known complete copy today is a single 11th century manuscript that was found behind a bookshelf in a monastery in the 19th century.
I can't speak to the reasons for the loss of Parmenides' poem, but scholars seem to think that the Nag Hammadi was indeed buried to prevent their destruction after Athanasius condemned non-canonical sources https://en.wikipedia.org/wiki/Nag_Hammadi_library.
Regardless, my point is that at least a written text has the chance of being discovered and understood in the future. The arctic code vault is actually interesting in this regard, but decoding it to find the jewels would likely be extremely challenging. There also is a lot of important technical information that isn't on github.
The point, though, is these texts were doomed when there ceased to be a scribal community that cared about them being copied. If you suppressed the heretics in antiquity, you suppressed the transmission of heretical texts except to the extent those texts were quoted in refutations that scribal communities did care about preserving. Actual destruction of manuscripts was unnecessary at that point, although it still sometimes happened like with the works of Arius and Nestorius. At least with Constantine's condemnation of Arius, it seemed to be more of a damnatio memoriae than an a practical act in furtherance of suppressing heresy.
But they didn't do that because they were trying to destroy heretical thought. They did it because they didn't realize what they had, they were poor, and needed kindling to stay warm or cook food. That's an important difference. Many ancient texts have been lost this way not out of malice, but just because that's what happens throughout the ages.
> Because I don't know of any
Of course you haven't, by definition. We find out about the things that survive destruction, not the ones that don't.
edit:
As for the instinct to preserve knowledge, that's not all we have. We also have a profit-maximizing infection that spread to every organ, so to speak. Books printed today don't last very long. Heck, not even newspapers and tech companies give a crap about link rot! Archive.org is a team of volunteers. Wikipedia is very hit or miss, and the ego of some seems to override whatever "instinct" we might have real quick.
So, frankly, I'm not seeing it. Even computing is rotting from under our hands. The Amiga came with schematics of the machine as part of the manual, while nowadays repair shops have to hunt for info. If companies were allowed, they'd each have their own incompatible charger cable, you bet. And let's not even talk about about 100MB applications that do 100KB things, because it's easier for the developer to never care about anything close to the machine and just use the bloated tool chain they already know. Recently I asked for a good (as in, other than "Microsoft wants you to") reason to use Win 11, and just got downvoted. Ever since Win 11 was announced I haven't gotten one answer to that question, ever. There's people who turn their nose up at it, and people who use it who don't want to talk about why. I could go on.
> No greater mistake can be made than to imagine that what has been written latest is always the more correct; that what is written later on is an improvement on what was written previously; and that every change means progress. Men who think and have correct judgment, and people who treat their subject earnestly, are all exceptions only. Vermin is the rule everywhere in the world: it is always at hand and busily engaged in trying to improve in its own way upon the mature deliberations of the thinkers. [..] An old and excellent book is frequently shelved for new and bad ones; which, written for the sake of money, wear a pretentious air and are much eulogised by the authors’ friends. In science, a man who wishes to distinguish himself brings something new to market; this frequently consists in his denouncing some principle that has been previously held as correct, so that he may establish a wrong one of his own. Sometimes his attempt is successful for a short time, when a return is made to the old and correct doctrine. These innovators are serious about nothing else in the world than their own priceless person, and it is this that they wish to make its mark.
-- Arthur Schopenhauer
the maya codices and the khipu were lost in very nearly their entirety, and the more abundant maya hieroglyphs carved on stone have been only partly deciphered. the khipu are entirely undeciphered except for arithmetic
of carthaginian literature not one book remains
the khwarazmian empire is known from the accounts of its neighbors and destroyers
qin shi huang ordered the burning of the history books for every kingdom he conquered; consequently what we know of most of them today is little more than legend, except for what the histories of qin tell us
in countless smaller-scale examples we don't even know the names of the nations that perished along with their language and books