Truthfully, i really just think it's because Itachi knew he couldn't actually kill Orochimaru. Itachi is strong. Everyone knows this. But at this point in the manga i'm inclined to believe that it would take nothing short of a full powered bijuudama, or perhaps Madara's perfect susano'o, or something in Hashirama's arsenal of equal power to actually kill him.
I don't think, as intelligent as he is, that he could have ever predicted Orochimaru trumping the Totsuka blade. As far as he knew, it was the only sure-fire way to get rid of Orochimaru for good. Then again I assume he also never predicted that Sasuke would go and revive Orochimaru either...