أَن تَقُولُوٓا۟ إِنَّمَآ أُنزِلَ ٱلْكِتَـٰبُ عَلَىٰ طَآئِفَتَيْنِ مِن قَبْلِنَا وَإِن كُنَّا عَن دِرَاسَتِهِمْ لَغَـٰفِلِينَ Qur’an Al-An'am (6:156)An taqooloo innama onzila alkitabu AAala taifatayni min qablina wain kunna AAan dirasatihim laghafileena
I.e., to the Jews and the Christians, who were the only two communities known to the Arabs as possessing revealed scriptures.
Because the diligent studies of the earlier People of the Book were in languages foreign to the new People of Islam, or because they were meant for circumstances different from those of the new world after Islam.