The typical ancient city consisted of a central population with a large amount of farmland associated with it. Often, people might live in the city, but still go to their fields every day. Many others lived in that farmland. Therefore, when Jesus had appeared to those gathered in the city, those who were in the farmlands would not have been aware of the important event happening in the city.
Such was the joy that was felt; it needed to be shared. Jesus would return on the next day, and there were probably many in the city who were related to those who lived in the farmlands. Therefore, they sent messengers to bring those people in. Since we never hear of any mode of transport, other than walking, in the Book of Mormon, the distance they could go was limited by how far they could walk in the available time. Even though that might be perhaps twenty miles in a day, they had to go and return. The messengers would not want to miss Jesus’s return.
Thus, while many were sent out, they would not have reached a different city. At least in Mesoamerica, even very large cities tended to be a 3-day walk apart. Perhaps there was a small village that was within reach, but if the gathered were “an exceedingly great number,” it would have been less than twice the first day’s crowd of two thousand five hundred (3 Nephi 17:25).