Not that it effects you're point, but Assange is an Australian citizen.
Rather than fight the UK on a hacker extradition again, the US seems to've taken advantage of the convenient fact that their target voluntarily placed himself on US soil.
*The core of the McKinnon fight seems to have been the fact that he was offered a chance at a much-reduced sentence if he cooperated and provided information about what he'd done and how he broke in, but on the UK side that was spun as a form of extortion (in other words, rather than a "be helpful and you'll get a lighter sentence" they read it as "fight us and we'll try to lock you up forever").
These extradition fights are largely played out in the media, it would probably have been unwise for the US to request the extradition of another sympathetic character with lots of community support behind them.