In my mind it's a numbers game. Evil God cults are going to be a relatively small subset of a civilization's population. Otherwise, it stops being a functional civilization. That lowers the number of people to write books.
Such cults are proscribed, and being caught with such material is unlikely to end well for the owner. That reduces the motivation to write said books, increases the incentive to destroy them, and ensures those few books that do survive are much harder to find. Plus, there's the Muslim conquest (assuming that it had already happened when Horra was there).
Of course, less probable does not mean impossible. Horra could have gotten ridiculously lucky. Or some dark spirit guided him to that book in service of dark designs that Horra may or may not have gone along with. I just think other possibilities are more likely.
Thank you. All that time reading Ars Magica sourcebooks is finally coming in handy.