Orochimaru joined Akatsuki for the sole purpose of gaining Itachi's body, to gain his Sharingan. The attempt of course failed and this made him a traitor to Akatsuki. Orochimaru then left because he felt threatened and he knew Itachi was a lost cause. It has been shown that Akatsuki does not tolerate betrayal to the group. An example of this is when Obito tries to kill Kabuto for betraying the organization, as Kabuto was once an Akatsuki spy working under Sasori, and the said they would deal with Orochimaru after the Tailed Beast Hunt.