I'm wondering whether Garner was put in there- to take a bullet. And since he has proven it's "safe", the replacement can be a bit more comfortable.
Or I also wonder if Garner blew it in some ways- perhaps in term of some relationship to the Kurds (he might have been too close to the Kurds), or that thing to do with Baghdag's mayor.
Also another factor could have been "career risk"- Garner probably didn't have high hopes in this area, but this new diplomat might have not wanted to risk the rest of his career, so with success more likely now, is a better time to get in there, but he can get credit for "fixing (maybe nonexistent) mistakes.