The importance of preserving a tribe, of the twelve Tribes of Israel, was an object, to which all Israel had regard. Perhaps the thing was from the Lord. So important is it, that even the New Jerusalem, is said to have their names on its gates. Revelation 21:12.
Related Commentaries of Judges 21:5
Judges 21:5-6
5 And the children of Israel said, Who is there among all the tribes of Israel that came not up with the congregation unto the LORD? For they had made a great oath concerning him that came not up to the LORD to Mizpeh, saying, He shall surely be put to death.
6 And the children of Israel repented them for Benjamin their brother, and said, There is one tribe cut off from Israel this day.