Per the Book of Moses read in Nehemiah 13, why were Ammonites and Moabites barred from God's assembly?

Answer: They hired Balaam to curse Israel

They had not met Israel with food and water but hired Balaam to curse them, though God turned the curse into a blessing.

“At that time the Book of Moses was read aloud in the hearing of the people, and in it they found the passage stating that no Ammonite or Moabite should ever enter the assembly of God,”— Nehemiah 13:1 (BSB)