These four verses provide all the information that we have about Alma’s preaching in the land of Melek. We do not know why Mormon did not include the quoted sermon. Perhaps it had no significantly new information. The only information we have is that he taught, and many were baptized.
From Melek, Alma journeys to Ammonihah. In Ammonihah we will have a mixture of quotation and narration, because it was an important incident and example. Mormon will spend longer on the events of Ammonihah than on any of the other sermons.