Q. Why did Jephthah have to sacrifice his daughter to God?

A. He had made a vow to God that if God helped him defeat the Ammonites, whatever came from the his house to meet him, it would be the Lord's.

And Jephthah made a vow to the Lord and said, "If you will give the Ammonites into my hand,then whatever comes out from the doors of my house to meet me when I return in peace from the Ammonites shall be the Lord's, and I will offer it up for a burnt offering."

Judges 11:30 ESV.

For the full story, go to Judges 11. It explains a lot more.