Because the Man'ari are lieutenants among the Legion. And Draenei do not want anything to do with fel or demons or any of that. It ruined their planet and every other planet they go to. Why would they want to side along something like that?
EDIT: i saw another person say "maybe for horde". The whole idea of ANY playable faction siding with someone who is/was a high-ranking member of the Legion is laughable.