The short of it is that Alexandria, including its libraries, declined as a center of scholarship after Ptolemy VIII exiled or killed all the intellectuals who worked there in 145 BCE. While the library survived, subsequent administration didn't maintain the institution. Whatever insults the physical institution suffered or when the buildings were finally destroyed, the institution of legend died from neglect long before then.
> subsequent administration didn't maintain the institution
Also papyrus has a limited shelf life, so even the books need to be maintained (copied) to maintain them in the long run, so if the institution is failing, the books will be lost in the medium/long term, even without direct destruction of the library
To contextualize why "history for atheists by an atheist historian" is a thing...
> After over ten years of seeing supposed “rationalists”, most of them with no background in or even knowledge of history, using patent pseudo history as the basis for arguments against and attacks on religion, I felt someone needed to start correcting the popular misconceptions about history which are rife among many vocal atheist activists. I also felt there needed to be some push-back by a fellow unbeliever against several fringe theories and hopelessly outdated ideas which have no credibility among professional scholars and specialists, but which seem to be accepted almost without question by many or even most anti-theistic atheists.
It’s surprising to me that the author finds it improbable that we don’t know many ancient writers. It’s always been my default assumption that the vast majority of all works and writers of antiquity were lost. I would expect that, like today, there is a relatively small number of hugely popular works, which had a decent chance of surviving, and a massive long tail of less popular works that were relatively obscure, especially over the time scale, didn’t get copied, and so therefore were lost. Of course we might get a few random selections from the long tail, but not many compared to the number of total works. I would have supposed there was easily a ratio of 100:1, at least.
I've spent my whole life believing that it was a disaster when the library at Alexandria burned. Is it possible it wasn't a big deal? How do we know?