I am sure it is because he is referred to The Word come to Life, as in the old testament prophesies of him have come to flesh he is The Word that was spoken of. Couldn't give references as I am not 100% with my Bible at the moment but I am sure that is the reasoning behind it.
There have been many opinions about the reason why this name was given to the Son of God. Those opinions it is unnecessary to repeat. The opinion which seems most plausible may be expressed as follows:
1st. A word is that by which we communicate our will; by which we convey our thoughts;
2nd. The Son of God may be called "the Word," because he is the medium by which God promulgates his will and issues his commandments.
It is through Jesus that we come to understand the true nature of the Father. He said, "if you have seen Me, you have seen the Father." This is why He is also called the Wisdom of God in the Old Testament. It is through Jesus that God chose to create, redeem and continue to deal with the world, including glorifying the church. If you consider the perfect integrity of the mouth of God, you see that calling Jesus the Word of God is a powerful testimony to His deity. Some biblical reflections:
Long ago, at many times and in many ways, God spoke to our fathers by the prophets,
but in these last days he has spoken to us by his Son, whom he appointed the heir of all things, through whom also he created the world.
He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. After making purification for sins, he sat down at the right hand of the Majesty on high, -Heb 1:1-3.
He is the image of the invisible God, the firstborn of all creation.
For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities-all things were created through him and for him.
And he is before all things, and in him all things hold together.
And he is the head of the body, the church. He is the beginning, the firstborn from the dead, that in everything he might be preeminent.
For in him all the fullness of God was pleased to dwell,
and through him to reconcile to himself all things, whether on earth or in heaven, making peace by the blood of his cross. -Col 1:15-20
The term "Logos" had already been used by philosophers in NT times, although John 1:1 gives "Logos" a different meaning.
__________________ Lord, make me an instrument of your peace on this forum. Where there is hatred, grant that I may sow love; where there is injury, pardon; where there is discord, unity; where there is doubt, faith; where there is error, Your truth and not my personal opinion; where there is despair, hope; where there is sadness, joy; where there is darkness, light. O, Divine Master, grant that I may not so much seek to be consoled by people here as to console them; to be understood by them, as to understand them; to be loved by them, as to love them: For it is in giving that we receive; it is in pardoning that we are pardoned; and it is in dying that we are born to eternal life.
Jesus is called The Word because the Greek word "logos" encompasses a lot...if you study the word "logos" you'll find it encompasses so much that many definitions fall short.
Here are some meanings of the word "logos" :
Adam Clarke's Commentary On The NT:
Logos, which signifies a word spoken, speech, eloquence, doctrine, reason, or the faculty of reasoning...
New Commentary On The Whole Bible NT:
Word—Greek, ho logos, signifying primarily “the Expression”—God expressed, God explained, God defined (see John 1:18). The Greek term logos in philosophical terminology also denoted the principle of the universe, even the creative energy that generated the universe (Morris). Thus, Christ as the Logos is the agent of and the personal expression of the Creator God.
So basically Jesus is The Logos or "The Expression of God"...and there's just no words to define it completely.