What crime did Assange commit in the UK?
In order to be "extradited" you first have to be a criminal suspect or a convicted criminal, Assange is neither in the UK.
But, because of the EU treaties, any member of the EU can hold an individual for a crime that was committed in any other EU territory. In this way, because Sweden claimed criminal wrongdoing, the UK held Assange so that he could be extradited to Sweden to stand trial for the crimes alleged there.
Assange has to first go to Sweden before the US can ask for Extradition, because Sweden is the one alleging criminal wrongdoing. If the UK simply sent him straight to the US, it would be obvious that the whole thing was a farce as it would have been an illegal extradition because Assange is not wanted for any crimes in the UK.
The whole pretense for the arrest warrant issued to Assange was so that the Swedish Prosecution Authority could interview him on the particulars of the case. Assange had offered to be interviewed multiple times by Swedish Investigators, but they all declined. Making it pretty clear that the point of the arrest warrant wasn't to interview him, but to get him onto Swedish soil.
The question then becomes: why?