Israel has been in Mount Sinai for a year. The people have received the Law and the assurance of the presence of God in their midst, enthroned on the Ark of the Covenant. Now they are preparing to depart for Canaan.
b. One tribe was exempt from this Why?
Tribe of Levites was exempted because God has ordained them to attend to the Tabernacle. Following are the verses.
50 Rather you shall appoint the Levites over the tabernacle of the covenant, and over all its equipment, and over all that belongs to it; they are to carry the tabernacle and all its equipment, and they shall tend it, and shall camp around the tabernacle. 51 When the tabernacle is to set out, the Levites shall take it down; and when the tabernacle is to be pitched, the Levites shall set it up. And any outsider who comes near shall be put to death. 52 The other Israelites shall camp in their respective regimental camps, by companies; 53 but the Levites shall camp around the tabernacle of the covenant, that there may be no wrath on the congregation of the Israelites; and the Levites shall perform the guard duty of the tabernacle of the covenant. 54 The Israelites did so; they did just as the LORD commanded Moses.
The Holy Bible: New Revised Standard Version, Catholic Edition (Nu 1:50–54). (1993). National Council of Churches of Christ.