In addition to asking why the Book of Mormon is so long we can also question why it is so structurally complex. The Book of Mormon is an abridgment of multiple plates with interwoven commentary from its compiler Mormon. It is not simply a collection of books one after another in chronological order.
In total there are:
Compared with the New Testament, the Book of Mormon is much more complex. Why would Joseph take this risk? Why make it more difficult for himself to keep track of all the plates and records mentioned through the book?
It seems odd that if Joseph Smith was creating a fraud and dictating it to a scribe that he would make it so structurally complex. Doesn’t this structure sound more like a genuine historical account rather than the work of Joseph Smith? Isn’t the structure even more confusing knowing that Joseph likely translated Mosiah first (after the lost 116 pages) and then went back to translate Nephi at the end?