There was little demand for reprinting old books in a language that no one could read. I don’t remember when Anglo-Saxon texts started to be studied, but it is more recent than I expected. Besides destruction by fire, many old texts were scraped off the pages, so the pages could be reused.
Knowledge of Anglo-Saxon was on the edge of oblivion. Other minor languages from that era are virtually unattested.
Gothic, for example, is known almost entirely from two books, both translations of the Bible, a total of less than 400 pages. And Wikipedia says it is “the only East Germanic language with a sizeable text corpus”. That’s considered sizeable in this context.
Also, until the mid 20th century, the poem was viewed chiefly as a historical artifact, of interest only to medieval historians. The idea that it could be appreciated as a work of literature was championed by Tolkien, and he’s often said to have single-handedly founded modern Beowulf criticism.
What’s more Gothic is absolutely central to our understanding of the history of the whole Germanic sub-group, and to a lesser extent of the Indo-European language family. A clear case of “size doesn’t matter”.