Komentarz do Diwrej ha-jamim I 5:2

Because Judah prevailed over his brothers as it says (Gen. 49:9): “Judah is a lion’s cub.”

and the one appointed as prince was to be from him the kingship emanated from him, and for this reason the king was called נָגִיד, since he is the one who leads [the people] out and brings them in [the Aramaic root נגד means to pull or lead.] And the birthright and the throne were taken from Reuben, and the throne was given to Judah, as it is written: “Because Judah prevailed over his brothers”; but the share of the birthright was given to Joseph, to his two sons.