We seem to have much more early (ante-nicene) christian writing than sacred texts or commentaries from competing faiths like Manichaeism or Mithraism contemporary to those writers. Was this a matter of Christian civilization taking effort to preserve those texts? How much luck was involved?

Can you clarify a little regarding Manichaeism? There are a good number of Sogdian and Turkic texts from Bezeklik. There have been many fragments found at Kellis in Coptic, Syriac and Greek. There's an obscure text called "Sermon on the Great War" in, I believe, Coptic. There's the Manichaean Psalter, also in Coptic. Fragments of and numerous commentaries on Mani's text "Shapuragan" survive. Do you mean why they weren't continuously copied after Manichaeism had been persecuted and outlawed in the Roman Empire?