Why did Jacob refuse to give Reuben the birthright and blessing?
Jacob refused to give Reuben the birthright and blessing because Reuben, his eldest, had committed adultery with one of Jacob's wives. Though the sin was seemingly unknown to others, Jacob revealed it at his deathbed, declaring Reuben would not be esteemed as the eldest.
Les explains that when Jacob blessed his 12 sons, he started with Reuben, the eldest, and brought to mind that Reuben was the most unchaste and promiscuous and had done the most awful deed. He had committed adultery with one of Jacob's wives. Evidently nothing was ever said of it until old Jacob's deathbed. At that time Jacob declared that Reuben would not have the birthright or the blessing and would not be esteemed as the eldest because he committed adultery with one of his wives. Seemingly no one else ever knew it.