Why does Joseph get the birthright and Judah the prince?

The chapter gives one reason, and it is about Reuben: "because he defiled his father’s couch, his birthright was given to the sons of Joseph" (5:1). It doesn't say why Joseph's sons received it rather than another brother. Of Judah and Joseph it says only this: "For Judah prevailed above his brothers, and from him came the prince; but the birthright was Joseph’s" (5:2). Genesis has Jacob's words to both. Of Joseph's sons: "Ephraim and Manasseh, even as Reuben and Simeon, will be mine" (Genesis 48:5), and to Joseph, "I have given to you one portion above your brothers" (Genesis 48:22). To Judah: "Your father’s sons will bow down before you" (Genesis 49:8), and "The scepter will not depart from Judah" (Genesis 49:10). The law of Deuteronomy 21 gives the firstborn "a double portion of all that he has", and ends, "The right of the firstborn is his" (Deuteronomy 21:17), with the same Hebrew word as birthright here (tap birthright). Chronicles doesn't join these verses together, and neither does this study.