“Yes, all Israel have transgressed your law, even turning aside that they should not obey your voice. Therefore has the curse been poured out on us, and the oath that is written in the Law of Moses the servant of God. For we have sinned against him.”
Daniel points back to the written Law. Remember his reference to Deuteronomy earlier. They have broken God's Law. And they have also refused to listen to the voice of God through His prophets. That is why they have been cursed, as indeed God had warned them that they would be (Jeremiah 44:22; Deuteronomy 27:26; Deuteronomy 29:20; and in detail Deuteronomy 28:15 onwards; Leviticus 26:14 onwards).