The Nephites are new at the concept of democracy. Although kings in the past had sought for the voice of the people (Mosiah 7:9; 22:1), they had just recently absolved the monarchy of king Mosiah and established a system of judges (Mosiah 29:25). In this instance, the chief judge, (possibly Nephihah who was appointed to fill the absence when Alma abdicated that responsibility, see Alma 4:17-19) doesn't take it upon himself to make the decision. Wisely, he gives this responsibility to the people.
Interestingly, he does not give them a proposition on which to vote "for" or "against." He leaves the entire proposition up to the people. They, as a group, come up with the idea to give up the land of Jershon for their brethren. This kind of democracy lays the responsibility at the feet of the people. Having made the decision themselves, they are much less likely to complain about the Anti-Nephi-Lehies when things get difficult.