Why? Jephthah had no son, and his only daughter would never be able to marry and bear him grandchildren.
فَهُوَ عَرَفَ عَلَى ٱلْأَرْجَحِ أَنَّ ٱبْنَتَهُ سَتَكُونُ أَوَّلَ مَنْ يَخْرُجُ لِمُلَاقَاتِهِ .‏