“As long as you live, you must never promote the welfare and prosperity of the Ammonites or Moabites.” -Deuteronomy 13:6
The Ammonites and Moabites weren’t just different from the people of God, they were hostile. For generations, they intentionally refused any help or assistance, and in some ages even attacked and oppressed the Israelites. Because of this, God issued this blanket command to His people that they were to offer no help to these enemies of the Lord.
That’s one of many reasons I find the story of Ruth so encouraging.