In direct contrast to the convention of the wilderness, in which the Pillar of Cloud and Ark always precede the people to their destination, here the Pillar of Cloud has in fact disappeared completely and the Ark remained temporarily behind, to indicate to the people that settling Canaan will require a higher level of involvement on their part. They have not been guided to the banks of the Jordan only to maintain the helpless object status that characterized them at Yam Suf. Here, they must complete the process of their liberation by demonstrating direction, resolve, and enterprise. Therefore, they must pass before the Ark. 

Courtesy of the Virtual Beit Midrash, Yeshivat Har Etzion