Why were Ephraim and Manessah given tribe status?

One thought:

Reuben was the 'first born' to Jacob(Genesis 49:3), and so was entitled to the rights and responsibilities that went with the honor, including TWO portions of his father's estate, but because he acted recklessly, and disgraced his father, his 'first born' rights were removed(Genesis 49:4)(Genesis 35:21+22) and given to faithful Joseph, through his two sons, Ephraim and Manasseh(1 Chronicles 5:1/Genesis 48:5).

Another Answer:

They were adopted by Abraham:

Genesis 48:5New King James Version (NKJV)

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