Who? The Americans? They can't extradite him from the UK because Sweden put in a EAW first. They can't request extradition from Sweden because he's not there yet. Anyway, if they want to extradite him, it will look a lot better if done after some extra dirt has been dished on Assange in the Swedish courts. If I were a US prosecutor, I wouldn't be in any rush.
I'm not saying that is whathappening, only that it's feasible. It's one thing to think that Sweden will act reasonably in this case. It's quite another to expect the USA to do so.