The choice of a Slavonic term for the Greek word that opens the fourth Gospel was one of the hardest decisions the translators faced, and it left a mark on the language that has never been removed.
The Greek term carried at once the sense of a spoken word, of reason and of ordered account, a range built up by centuries of philosophy before any Christian used it. Slavonic had no word doing that work. The translators took slovo, the plain everyday noun for a word or an utterance, and let the theological sense accumulate around it through use, which is what had happened in Greek in the first place.
The decision is characteristic of the whole enterprise. Faced with a technical vocabulary that did not exist in the target language, the Thessalonican brothers and their successors preferred to press ordinary Slavonic words into new service rather than to transliterate Greek, and where they did transliterate, they did so sparingly. That policy is why Slavonic theological vocabulary is largely transparent to a Slavic speaker in a way that Latin derived theological vocabulary is not transparent to an English one.
It also has a cost, and the tradition knew it. A common word carrying a technical sense can be misread as its common self, and much later Slavonic commentary is occupied with keeping the two apart. The letter named slovo therefore sits at the point where an alphabet, a translation policy and a doctrinal term meet, which is more than most letters are asked to carry.