But even at that, Jacob feels a strong need to admonish some of his people to "despise not the revelations of God." This tells us that there must have been people there who were despising the revelations. Jacob thinks the best way to get them to not do that anymore is to just lay out for them the entire text of Zenos’ Allegory of the Olive Tree from the brass plates. They may not have read or heard this text recited very often in such complete detail.
Jacob even seems concerned that he might fail in his task: "I will unfold this mystery unto you; if I do not, by any means, get shaken from my firmness in the Spirit, and stumble because of my over anxiety for you" (v. 18). This makes me wonder if he was reciting Zenos’ allegory from memory. Whatever the case, the people likely didn’t have their own copy of the scriptures. So maybe the best thing Jacob could do to get them to not despise the scriptures was to just let them hear the whole thing and be impressed with the magnitude and the profundity of this elaborate allegory.
Book of Mormon Central, "Why Did Jacob Share the Allegory of the Olive Tree? (Jacob 4:17)," KnoWhy 66 (March 30, 2016).