1) Joker is a cool, terrifying villain, but he's a wild dog. I don't think that anyone would trust him on the team. Nor do I think that he would want to join despite the fact that he has in the past? He hates schemers. Why would he want to join? He doesn't want to kill Bats--just torture him to the brink. He would probs betray the LOD. Would it make more sense for Deadshot, Bane, Clayface, Hush, or someone else to join?
2) Sinestro is committed to his own brand of JUSTICE in the universe. Why would he choose to work with villains? Especially if he's not in charge? Would is make more sense for Atrocitus or Larfleeze to join?