If this is a truth question, one can only speculate because as far as I can tell, it does not appear that Joseph Smith ever addressed the question of why the Book of Mormon uses the English style it does.
Ultimately, the LDS perspective is that God commanded and helped Joseph to translate the plates so that translation was not decided by Joseph Smith himself, but by God. See Book of Mormon Translation - lds.org
The assumption that the Book of Mormon is written with the English of the Jacobean era stands to be proven. Comparing it to the KJV of the Bible may not necessarily be considered Jacobean English because scholars have said that 90% of it was based on the Tyndale translation which was done prior to the Jacobean period. Coggan 1968, pp. 18-19.
English in the Book of Mormon
Many of the stylistic examples in the Book of Mormon that may be incorrectly attributed to the style of English may actually be attributed to the original language of the text. For example,
Possession in English is shown in two constructs-"the man's house" and
"the house of the man"-but only the latter construct is employed in
Hebrew. The lack of apostrophe possession in the Book of Mormon is
consistene with a translation from the Gebrew construct. Further, the
"of" construct is common for adjectival relationships in Hebrew.
Correspondingly, the Book of Mormon consistently employs phrases such
as "plates of brass" (1 Ne. 3:12) instead of "brass plates" and "walls
of stone" (Alma 48:8) rather than "stone walls."
Sentence structures and clause-combining mechanisms in Hebrew differ
from those in English. Long strings of subordinate clauses and verbal
expressions, such as those in Helaman 1:16-17 and Mosiah 2:20-21 and
7:21-22, are acceptable in Hebrew, though unorthodox and discouraged
in English: "Ye all are witnesses…that Zeniff, who was made king,…he
being over-zealous,…therefore being deceived by…king Laman, who having
entered into a treaty,…and having yielded up [various cities],…and the
land round about-and all this he did, for the sole purpose of
abringing this people…into bondage" (Mosiah 7:21-22).
Many more examples are given in this Study done on Book of Mormon Language
A quick text analysis of the Book of Mormon shows that the phrase "and it came to pass" occurs in the Book of Mormon 1123 times. One LDS member makes the case that the phrase "and it came to pass" is precisely what certain words in the original language means (therefore not necessarily an english phrase from an earlier era). See "And it came to pass..."