Viro Litterarum salutem multam.
Perhaps the first question to be asked in order to dillucidate this subject is: Who was that man, writer of the fourth gospel, that Ecclesiastical tradition has named John (Ioannes)?
If we consider some parts of the fourth gospel, we could say that so-called John appears to be familiar with Gnostic doctrines. And if we remember that Gnosticism is based, at least partially, on a "miscelanea" of Greek philosophy, we see where the use of "logos" most probably come from: from the Greek philosophical doctrines about the "principles" ("principia", "arkhai"), which were supposed to form and preserve the world. Heraclitus mentions that "logos" in his surviving fragments as the principle par excellence.
Thus, "logos" belonged to the traditional language of philosophical doctrines, and was only natural that "spiritual" writers (including our "John") used it instead of apparent and available synonyms such as "epos", "rhema", and others.
Hominibus totam versandam constat esse bibliothecam, ut solam utilem scribere sententiam possint.