Haaretz asks: "Why is Dennis Ross being ousted as Obama envoy to Iran?"
1. Ross, a veteran diplomat, was never envoy to Iran. He was a "special adviser" with responsibility for providing advice about the entire Middle East region (except for, um, a lot of other countries in the Middle East.) The U.S. government never wanted an envoy to Iran because an official appointment would signify that Iran merited the same official attention and respect as, say, Afghanistan and Pakistan. Obama's engagement policy is more cautious than that.
2. His position at the State Department has been tenuous and his status unclear; that's led to leaks and questions from his friends about what the heck he does.
3. He remains a valued adviser to the president, and he will probably end up serving in an advisory capacity at the National Security Council, where he won't have to fight political battles with Richard Holbrooke (who has hired Iranian experts for his staff) and others.
4. Ross may end up coordinating Iran policy at the NSC.