Exodus 6:14

These are the heads of the house of their fathers. The sons of Reuben, the firstborn of Israel, Hanok and Phallu, Hezron and Karmi; these are the race of Reuben.

THESE ARE THE HEADS OF THEIR FATHERS’ HOUSES: THE SONS OF REUBEN…. Scripture did not wish to begin with the statement, And these are the names of the sons of Levi according to their generations, so that it should not appear that henceforth in honor of Moses, Levi is first in genealogy. Instead, [Scripture therefore] mentioned Levi’s elder brothers, [i.e., Reuben and Simeon], and that Levi is counted third.
Now Scripture mentioned the sons of Levi according to their generations because in the case of Reuben and Simeon, it mentioned only those who went down to Egypt with them, but in the case of Levi it mentioned his children, the number of years of his life, the birth of the fathers of the prophets — [namely, Kohath the grandfather, and Amram the father of Moses and Aaron] — and the number of years of their lives, all in honor of the prophets. Besides, they themselves — [Kohath and Amram] — were the pious ones of the Most High, worthy to be spoken of as the fathers of the world.

אלה ראשי בית אבותם, this is explained in the Mechilta as a reference to the three (potential) tribes Reuven, Shimon, and Levi, whom Yaakov had chastised on his deathbed. At this point the Torah enumerates their genealogy to demonstrate their importance in the fabric of Jewish nationhood. According to the plain meaning of the text, the Torah had to provide details of the tribes’ genealogy as far as Moses and Aaron, whereas it also had to provide details as far down the line as Korach, and the sons of Uziel; as well as Pinchas all of whom are mentioned by name and deed in the Torah later on. If we had not heard about their roots in this chapter we would not have known who these people had been.

אלה ראשי בית אבותם, Moses and Aaron were justifiably referred to as the heads of the various tribal groups of Israel, as they were the most respected and most honoured. ראובן בכור ישראל, seeing that none of his descendants were particularly noteworthy except his sons who had been part of the original 70 persons who had come to the land of Egypt from Canaan, none other are mentioned. All those 70 persons had already died as we know from Exodus 1,6. The same consideration held true for the descendants of Shimon. On the other hand, Levi, the longest surviving member of the original 70 Hebrews who migrated to Egypt, managed to raise even his grandchildren to become leaders of the people in their own right. Amram produced three outstanding children in the persons of Aaron, Moses and Miriam, all of whom attained prophetic stature. Aaron married a sister of Nachshon who had been considered the most notable member of the nation in his time. He begat leaders of the generation who were later on appointed as priests. Eleazar, son of Aaron took as a wife one of the daughters of Putiel who was also considered outstanding in his generation (compare Nachmanides). This union produced Pinchas who was granted a covenant of “peace” by G’d as a reward for spontaneously slaying a high ranking Israelite prince who flagrantly engaged in forbidden sex with a Midianite princess and had challenged Moses to do something about it. (Numbers 25,12) אלה ראשי אבות הלוים, just to tell us that Moses and Aaron were the respective heads of the Levites.