I'm reading the commentary on a Qur'an program and it says:

"This is the only sura of the Qur'an to which Bismillah is not prefixed. Though the commentators have given different reasons for this, the correct one that which has been given by Imam Razi: namely, this is because the holy Prophet s.a.w himself did not dictate it at the beginning of the sura. Therefore the companions did not prefix it and their successors followed them. This is a further proof of the fact that utmost care has been taken to keep the Qur'an intact so that it should remain in its complete and original form"