- Much that was written was lost due to persecutions.
I want to highlight this reason by way of example.
Among the early references to the Assumption in Christian literature, one of them refers to other authors whose works now seem to be lost. The passage I’m referring to comes from Eusebius’ Chronological Tables in Year 48 A.D. You can view the Latin of one manuscript here:
books.google.com/books?id=DxQRAAAAYAAJ&pg=PA584#v=onepage&q&f=false
Look on that page, near the top, where there is a chunk of text indented and centered, with the word “Romanorum” above it. On the right of that chunk of text you can see a table of years: “An. Dom.” is at the top, with the numbers 47, 48, 49, and 50 going down beside it. Look next to Year 48 and you’ll see the following text in Eusebius’ book: “Maria virgo Jesu Christi mater ad Filium in caelum assumitur, ut quidam fuisse sibi revelationi scribunt.” According to the translation
here, this does not appear in all manuscripts, and it translates to, “The virgin Mary, mother of Jesus Christ, is taken up to her son in heaven, as some write had been revealed to her.”
If the entry is a genuine text of Eusebius, that makes this a very early explicit reference to the Assumption from the early 300s A.D. And it is very interesting that the text refers to other authors: “as some write had been revealed to her.” Who are the “some”? Well, we do know some of Eusebius’ sources: Eusebius wrote his history based on books he consulted at the lost library at Alexandria. He paid special regard to the now-lost history by Hegesippus. At the beginning of his Chronological Table he names several sources: “Clement, and Africanus, and Tatian from among ourselves, and Josephus and Justus from among the Jews.”
The historical writings of all these authors are lost, except Josephus’s books. But if the Eusebius entry is genuine, then he knew of authors prior to himself, prior to the early 300s, who referred to Mary’s Assumption. Thus there is some reason to suspect that Mary’s Assumption was a known historical fact in the pre-Nicene Church, but the writings which refer to it are basically lost.