As a non-trinitarian (there are many of us), I have to come at this question with a non-trinitarian point of view and obviously give, at the very least, a somewhat believable, if not more than believable answer.
In John 1:1, we have the subject Jesus (the Word) and we have, believe it or not, God as the object within the sentence. We are told that... In (the) beginning was the Word... The second (non-imaginary) "the" is in the singular, masculine, nominative case, whereas the third (non-imaginary) "the" that comes before the first "Theos", with a capital "T", as this clearly refers to the "Almighty" himself, but is NOT the subject of the sentence, is in a singular, masculine, accusative case and therefore the object of the sentence. We are also told that... the Word was toward/with the "Almighty"... and the verb precedes the noun. Then we are told that the second "theos" (Oeog, as opposed to Oeov)...was the Word... and the verb comes after the noun (more on the positioning of the verb later). So we have two different applications of "Theos/theos", one preceded by the verb (was) and the other (was) succeeds the second "theos". The first "Theos" being the object of the sentence and the second "theos" being the subject. Consequently, we would seem to be talking about two separate divinities here, otherwise, the point of the two different applications would be lost to the absolute meaning. Most of us non-trinitarians are therefore of the opinion that the first "Theos" refers to "the God/the Almighty" and that the second "theos" refers to (a) god/Jesus, hence the reason for the second "theos" being without a capital "t", although the divinity of both should not be in question.
So, to get back to the question at hand: "Why is the Word in the beginning but God is not". Well the non-trinitarian answer to that is that God/Almighty has no beginning. JHVH means:- Proved to Be, or, Causes to Become... He caused Jesus to become (become the Only Begotten of God), the firstborn of all creation; and therefore have a beginning. Jesus means:- JHVH's salvation... and subsequently became the means for deliverance from sin and a short while later, the firstborn from the dead.
About the positioning of the verb (was). See the Appendix 2A, page 1139, of "The Kingdom Interlinear Translation of the Greek Scriptures, which also talks about other translations using the indefinite (god), as opposed to the definite (God) for the second "theos".
ERNEST CADMAN COLWELL RULE:
While Colwell's rule merely permits but does not demand that a predicate nominative ahead of an equative verb be translated as definite (God) rather than indefinite (god), it all depends on the context. If the latter part of John 1:1 were interpreted to mean "the" God, this would then contradict the preceding clause, which says that the Word was with God. It stands to reason, therefore, that the translation should be indefinite and more to the point "a" god, even though there was no indefinite article in Koine Greek language, but there was a definite article "the" and the second "theos" was not preceded by the definite article.
In Rev,3:14:- The Son, Jesus, is being referred to as... "the Amen (So be it, surely, truly, verily. Root meaning: A.man...be faithful, or trustworthy), who made singular use of the expression often.... the faithful and true witness, the beginning of the creation of/by God"...God's first creation in other words.
There is NO DIFFERENCE in John 1:1 and Rev,3:14 as to the meaning of the beginning. The "Amen" and the "Word" are both referring to God's first creation, that of the Only Begotten spiritual Son of God, through whom all other things (other than himself) were made. God may well have been the "Architect" of all things but Jesus was his "Contractor". See Proverbs 8:22-31, (NWT of the HS), an uncorrupted translation of the Hebrew, if there ever was one.....