A discussion on why the Nephites quoted Isaiah so extensively and, in addition, some keys to understanding Isaiah may be found in Commentary 1:273-77.
“Great Are the Words of Isaiah”
It is one thing for the prophets or the Saints to quote the Lord-such is appropriate and necessary. It is quite another for the Lord to quote someone and then to command the Saints to search that prophet’s writings! what greater recommendation could there be for us to begin a life-long search and study of Isaiah?“For Great Are the Words of Isaiah”
It just may be that my salvation (and yours also!) does in fact depend upon our ability to understand the writings of Isaiah as fully and truly as Nephi understood them. For that matter, why should either Nephi or Isaiah know anything that is withheld from us? Does not that God who is no respecter of persons treat all his children alike? Has he not open us his promise and recited to us the terms and conditions of his law pursuant to which he will reveal to us what he has revealed to them? (“Ten Keys to Understanding Isaiah,” Ensign, October 1973, p. 78)