Why was the birthright of Reuben given to Joseph?

De 21:

17 He shall acknowledge the firstborn, the son of the unloved, by giving him a double portion of all that he has, for he is the firstfruits of his strength. The right of the firstborn is his.

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By law, the firstborn received a double portion of inheritance from his father.

1 Chronicles 5:

1 The sons of Reuben the firstborn of Israel (for he was the firstborn, but because he defiled his father’s couch, his birthright was given to the sons of Joseph the son of Israel, so that he could not be enrolled as the oldest son; 2 though Judah became strong among his brothers and a chief came from him, yet the birthright belonged to Joseph)

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Reuben forfeited his firstborn right (Ge 49:4) because he slept with Bilhah, Jacob’s concubine (Ge 35:22). However, the firstborn right did not go automatically to the next in line. Jacob gave it to Joseph. Ge 48:

5 Now your two sons, who were born to you in the land of Egypt before I came to you in Egypt, are mine; Ephraim and Manasseh shall be mine, as Reuben and Simeon are."

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Under Joshua, both Manasseh and Ephraim received tribal allotments along with ten of Jacob's sons.

Why did Jacob give Joseph the firstborn right?

Reuben the firstborn by Leah, disqualified himself. Joseph was Rachel's firstborn. Jacob loved Rachel more than Leah. Joseph was Jacob's favorite son, Ge 37:

3 Now Israel [Jacob] loved Joseph more than any other of his sons, because he was the son of his old age; and he made him a robe of many colors.

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Unlike his brothers, Joseph was a righteous and more faithful son. He had not committed serious sins. Simeon and Levi killed men in their anger and hamstringed oxen on a whim (Ge 49:6). Joseph's brother sold him to Egypt. Jacob recognized Joseph’s integrity and God’s favor toward him, which likely influenced his decision to elevate Joseph's two sons.