It is possible that the "did read, and caused to be read" phrase here is significant. It is quite possible that the numbers of people were beyond the reach of Mosiah's voice, and that messengers had to be sent to read the information to those who could not hear.
A question that may be asked of this situation is why Mosiah chose to read the accounts of Limhi and later of Alma.
We tend to forget in our world of information overload that the ancient world was typically bereft of both information and entertainment. The reading of these records was, for them, both an important means of communicating important information about the new people who were going to be their neighbors, but also a form of entertainment. The excuse to gather and to hear something new would have been tremendously entertaining as a diversion for the daily tasks of survival. In the absence of television, public orations were the news shows and perhaps even the soap operas of the day.
It is also quite interesting to contrast something that is not noted here with the similar gathering of the people for Benjamin's great discourse. When Benjamin had to speak to the people, he found that he had to build a tower (Mosiah 2:7). When Mosiah speaks to a presumably larger audience, he does not mention the building of a tower.
As was noted in the commentary on Benjamin's speech, it is possible that he was speaking in the temple courtyard of a temple that was being built, but which had not yet been finished, thus the tower was necessary to lift him higher than the congregation. In Mosiah's case, the "temple" would have been built. If we assume it to have been a Mesoamerican style temple, it would easily have provided the elevation for such a public discourse. Indeed, temple-pyramids were the focus of such public gatherings in Mesoamerica.