John Gill Commentary Judges 21:15

"And the people repented them for Benjamin, because that Jehovah had made a breach in the tribes of Israel." — Judges 21:15 (ASV)

And the people repented them for Benjamin
That they had destroyed all their women, and that they had saved no more of the daughters of Jabeshgilead, not a sufficient number to be wives to the Benjaminites:

because the Lord had made a breach in the tribes of Israel ;
by almost destroying one of them; for though this was done by the Israelites, yet by the permission and according to the will of God, and through his overruling providence.