The correct opinion why there is no Basmalah (saying Bismillah) between Surah At-Tawbah and Al-Anfal is that the Companions were uncertain as to whether Surah At-Tawbah was the same Surah as Al-Anfal or they were two separate Surahs, so they said: Let us make a separation between the two Surahs and not write the Bismillah.

The Quran did not come down from Heaven to the Prophet Muhammad (peace and blessings be upon him) all at once; rather the Revelation of the Quran came down to him from Allah piecemeal according to events and circumstances.