Another possibility is that Atatsuke was made of of 9 of the most powerful criminals who were determined to get tailed beasts. And that Orochimaru was the weakest person in the group.

Then Itachi shows up and Atatsuke is all, "Well, there's 10 of us now and Oro, you're the weakest, so no tailed beast for you."

And then Oro is all like, "Fuck!" And leaves.