No doubt there were many cities and villages in which Alma preached and of which we have little or no record. Mormon, our writer-editor, was striving to preserve those lessons and precepts which would be of everlasting worth to future readers. In some cases therefore, he did not seem to feel the need to repeat a concept or a lesson or a teaching experience over and over again. Rather, he provided one strong illustration (such as Alma’s discourse at Zarahemla) to make his point.
In this case, because the people of Melek responded positively and receptively to his gospel message, Mormon reserved the space on the plates for something else: there was a negative lesson to be learned from the people in Ammonihah.