Is there any particular reason why "kai Theós ēn ho Lógos" is translated "and the Word was God" and not "and God was the Word"?
Yes. This is a predicate nominative construction. That is, both θεὸς (God) and ὁ λόγος (the word) are in the nominative case, and they are joined by an equative verb (here, a form of "to be").
John 1:1 (NA28):
Ἐν ἀρχῇ ἦν ὁ λόγος, καὶ ὁ λόγος ἦν πρὸς τὸν θεόν, καὶ θεὸς ἦν ὁ λόγος.
In English, we generally distinguish the subject (S) from the predicate nominative (PN) in such constructions based on word order. Greek is characteristically loose about word order but shows other patterns for making this distinction. In this case, the subject can be identified if one of the two nominatives falls into any of the following categories:1
- a pronoun
- an articular noun (i.e. preceded by the article)
- a proper name
If both nominatives have such a tag, the analysis is more complex. However, in the case of John 1:1, only ὁ λόγος (the word) — an articular noun — fits.2 Thus, it is correctly translated as in the ESV (and nearly every English translation):
…and the Word was God.
You may (or may not) be interested in considering the semantic distinction between the S and its corresponding PN. Given that they are joined by what I have called an "equative" verb, you may wonder if they should be interchangeable. We know that in English they are not, or you would not have asked the question. Greek is similar in this regard. The most common relationship is what Wallace calls a subset proposition.3 The S is a narrower group within the broader PN. In John 1:1, "the Word" (S) comprises a subset of a larger category called "God" (PN).4
(See also this related question for a defense of the translation “God” rather than “a god.”)
1. Daniel B. Wallace, Greek Grammar Beyond the Basics: an Exegetical Syntax of the New Testament (Zondervan, 1996), pp 42-46.
2. Note that θεὸς (God) is not a proper name in Greek.
3. The less common relationship is that of a convertible proposition in which the equative verb essentially becomes an
= sign. This construction can be identified when both nominatives carry one of the grammatical tags marking the subject. This is not the case in John 1:1 (see note 2).
4. If you would like to try this out elsewhere, skip down 13 verses: "the word became flesh." Here this subset relationship is more obvious. "Flesh" is the broader term; it would make no sense to say, "flesh became the word."