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ANSWER: Because Jacob loved Joseph more than all his other brothers.
Now Jacob lived in the land where his father had sojourned, in the land of Canann. These are the records of the generations of Jacob. Joseph, when seventeen years of age, was pasturing the flock with his brothers while he was still a youth, along with the sons of Bilhah and the sons of Zilpah, his father's wives. And Joseph brought back a bad report about them to their father. Now Israel loved Joseph more than all his sons, because he was the son of his old age; and he made him a vericolored tunic. And his brothers saw that their father loved him more than all his brothers; and so they hated him and could not speak to him on friendly terms.