John Gill Commentary Judges 21:18

"Howbeit we may not give them wives of our daughters, for the children of Israel had sworn, saying, Cursed be he that giveth a wife to Benjamin." — Judges 21:18 (ASV)

However, we may not give them wives of our daughters
Though their case was so very necessitous and desperate:

for the children of Israel have sworn, saying, cursed be he that gives
a wife to Benjamin ,
(Judges 21:1) and therefore without the violation of their oath could not give any of their daughters in marriage to them: therefore some other way must be devised to help them.