We are not told why Benjamin did not appear with the other tribes at Mizpah. They may well have not been invited. But upon hearing that the tribes had gone up to Mizpah, they decided not to join them. Perhaps the tribe had already decided to stand with Gibeah against the rest of their nation.1
3 (Now the people of Benjamin heard that the people of Israel had gone up to Mizpah.) And the people of Israel said, “Tell us, how did this evil happen?”