The Mufassireen (commentators of the Holy Qura’an) mention few reasons why Surah Taubah has no Bismillah:
1. There is a possibility that Surah Taubah is part of the previous Surah, (Surah Anfaal). Hence writing Bismillah in the middle of a Surah would go against the Tarteeb (Sequence) of the Holy Qura’an.
2. In this Surah , Allah Ta’ala expresses his anger, wrath and Bismillah denotes peace and mercy, therefore Bismillah was not written.