Charles Ellicott Commentary Numbers 3:32

"And Eleazar the son of Aaron the priest shall be prince of the princes of the Levites, [and have] the oversight of them that keep the charge of the sanctuary." — Numbers 3:32 (ASV)

And Eleazar the son of Aaron the priest ... — By virtue, it seems, of the descent of Moses and Aaron from Kohath, the Kohathites had the most honourable portion of the service of the Tabernacle assigned to them; and therefore, as the priests belonged to the Amramites, one of the four families of the Kohathites, Eleazar, the eldest surviving son of Aaron, was chosen to have the oversight over the whole body of the Levites.