Nearly the whole of our Chapter 36 of Alma is devoted to Alma’s description of his conversion story to Helaman. John W. Welch has formatted that chapter as a long chiasmus [less commonly known as chiasm], or a poetic form that begins and moves to a central text, and then takes each point and essentially repeats it in reverse until the last line parallels the first. That is a complex and beautiful passage. Shiblon gets the same story in only four verses.
We should not suppose that Alma cares less for Shiblon. Alma’s conversion story was certainly known to his sons, and it appears in their blessings and father’s counsel as part of the moral lessons. Perhaps it was unnecessary to give the longer version to Shiblon, if Shiblon were actually present for the first recitation. If he was not, perhaps Alma simply wrote the incident more simply as he recorded it and Shiblon did receive the very same recitation. Clearly, Alma intended that the opening admonition to both Helaman and Shiblon be about the same topics. Perhaps they were, in their oral form, closer than what Alma eventually wrote down.