The narrator did not previously mention the oath. But that was intentional. Delaying mention of it until now brings the reader to feel a bit of the surprise that the Israelites themselves feel as the full consequences of their actions become evident to them. They had been apparently so intent on wiping out Benjamin that they forgot the oath. Their recalling of it now hits them hard.1
1 Now the men of Israel had sworn at Mizpah, “No one of us shall give his daughter in marriage to Benjamin.”