Because they forsook the covenant of Jahova, their God, and worshiped other gods, and served them.
That's the reason Jeremiah gave for the downfall of Judah. Listen to this ancient prophet, America. Wake up, and forsake evil ways.
The Rev. John R. Gunn writes, "We need to wake up to the evils, which today threaten the very life of our nation. We scarcely need to name these evils: intemperance; irreverence; irreligion; materialism; vulgar and worldly indulgence; and a growing disregard for the sanctity of the law. If these evils go on unchecked, the breakdown of the nation is inevitable."