Reuben was a terrible leader. In Genesis 42:37, he even offered to kill his own sons if he lost Benjamin in Egypt. What!?

In contrast, Judah offered his own life as security for Benjamin's, showing himself to be a transformed man, a true leader of his people. And when Jacob listened to Judah, acted in faith, and sent Benjamin to Egypt in v14, he didn't lose the two sons as he feared (Benjamin and Simeon), but gained one he thought already lost (Joseph).