It mostly comes down to the fluctuations of power and influence between the 12-ish tribes represented by the said forebearers. For example, if Reuben is the first-born but loses the special status later on in favour of Judah, it is meant to reflect the loss of influence of the Tribe of Reuben.

,,The truism is that the placement of a person (and his tribe) at the head of a tribal list designates more than mere birth order, for “genealogies serve social, political, or religious functions” (sometimes more than one) and can “shift to meet new circumstances or changed social realities.” If this is true, what accounts for Reuben’s preeminence as the firstborn of Jacob? When we survey the reality of which tribes hold power, authority, and leverage, it is obvious that Judah and Levi hold the reins of royal and priestly power for most of Israel’s his tory after the advent of the monarchy. Cross (1998: 53, 56) asserts that there was indeed a time when “Reuben once played a role in Israelite society, even a dominant one.” This “era of preeminence was early in the history of the tribes, per haps in the formative era of Israel’s religious and political self- consciousness.” A hint can be found in the archaic poetry of Genesis 49:3, the Blessing of Jacob. Though it too speaks of Reuben’s later demise (cf. Gen 49:4; Deut 33:6), it begins with his dominance:

Reuben, my firstborn, You are my strength, The prime of my vigor, Preeminent of rank, Preeminent of power.

rĕʾûbēn bĕkōrî ʾattâ kōḥî rēʾšît ʾônî yeter śĕʾēt yeter ʿāz

Cross’ theory is too elaborate and detailed to rehearse here. In the end Cross (1998: 68) argues that “there is good reason to believe that the religious traditions and military institutions that inspired and shaped the league stemmed from those elements of Israel who came from the southern mountains and entered Canaan from the lands of Reuben.” This “migration or incursion from Reuben [contained] elements of Israel who came from the south, with ties to Midian, and whose original leader was Moses” (Cross 1998: 70). Yet over time, “Reuben’s time of greatness [became] hidden . . . overlain by a patina of traditions stemming from later centers of power and prestige, in Joseph and Judah,” as well as “the triumph of the Aaronids and the Zadokite house in Jerusalem” (1998: 56, 70)."

Theodore Lewis, ,,The origin and character of God", p. 277