John Gill Commentary 1 Chronicles 24:2

"But Nadab and Abihu died before their father, and had no children: therefore Eleazar and Ithamar executed the priest`s office." — 1 Chronicles 24:2 (ASV)

But Nadab and Abihu died before their father
While he was living, see (Leviticus 10:1Leviticus 10:2)

and had no children, therefore Eleazar and Ithamar executed the
priests' office :
otherwise, as Jarchi observes, had their brethren left children, they would have come into the office before them.