Board Thread:Versus Debates/@comment-31568508-20170323180643/@comment-3965545-20170325203600

Itachi hadn't seen Kamui's intangibility ability except one time when he was 8 and Obito killed his teammate, so he never got the chance to analyze it. Obito said very, very clearly that if Itachi knew about Kamui more then Obito would be dead, no ifs or buts. It's in the 3rd Databook and in the manga.