Why were the Israelites not allowed to get close to the Ark when crossing the Jordan?
The Israelites were commanded to keep about 2,000 cubits—roughly 3,000 feet, or a little over half a mile—between themselves and the Ark. This distance let them clearly see the Ark and know the way to go, since they had not passed that way before. The Ark signified God's presence and leadership.
Les explains that the Ark of the Covenant was the sign of Israel's leadership and the very presence and mercy of God. It was the gold-overlaid box containing the tables of the Ten Commandments, a sampling of manna, and Aaron's rod that budded, with the mercy seat above it and the Shekinah glory above that in the Holy of Holies. Because it represented God's presence, they were never to get too close. Verse 4 specified a space of about 2,000 cubits, roughly 3,000 feet, or a little over half a mile, between the people and the Ark. The stated purpose was, "Come not near unto it, that you may know the way by which you must go, for you have not passed this way before." The distance allowed the people to see the Ark clearly as the divine guide as the priests, the Levites, bore it before them across Jordan.
Yet there shall be a space between you and it, about two thousand cubits by measure: come not near unto it, that ye may know the way by which ye must go: for ye have not passed this way heretofore.