Instead of all the first-born, who were God's propriety by right of redemption, Exodus 13:12, and to whom the administration of holy things was formerly committed, which now was taken away from them, either because they had forfeited this privilege by joining with the rest of their brethren in the idolatrous worship of the calf, or because they were to be mainly concerned in the distribution and management of the inheritances which now they were going to possess, and therefore could not be at leisure to attend upon the service of the sanctuary; which made it fit that this work should be committed to other hands. And God would not commit it to some other persons in each tribe, which might be an occasion of idolatry, confusion, division, and contempt of sacred things, but to one distinct tribe, which might be entirely devoted to that service, and particularly to the tribe of Levi; partly, out of his respect to Moses and Aaron, branches of this tribe; partly, as a recompence of their zeal for God and against idolaters; see Exodus 32:26,29 Deu 33:9; and partly, because it was the smallest of the tribes, and therefore most likely to find both employment in and maintenance for the work.
Related Commentaries of Numbers 3:12
Numbers 3:12
12 And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine;