You can see why from what else Jeremiah wrote about the heart. Some chapters of the book of Jeremiah dealt with nations round about, yet its main focus was the nation of Judah during the reigns of five of its kings.
حَتَّى لَوِ ٱتَّخَذْنَا كُلَّ هٰذِهِ ٱلْخُطُوَاتِ ،‏ فَلَا نَزَالُ بِحَاجَةٍ أَنْ نَنْتَبِهَ إِلَى مَوْقِفِنَا وَمَسْلَكِنَا .‏