John Wesley on Numbers 3:12

The first - born - Who were God's property, Exo 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: 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 against idolaters, and partly because it was the smallest of the tribes, and therefore most likely to find both employment in, and maintenance for the work.