Who knows. It is obvious why the US initially did not want to extradite Assange from the UK, because such an extradition would seem to have a low chance of success and even if it did work it would take a very long time. The UK may be a five eyes country but it also has a strongly independent judiciary.

To speculate, perhaps the US's original plan was to waiting for Assange to make his own way to a more pliant jurisdiction, and the whole Sweden case caught the US by surprise as well.

As soon as Assange ended up in the Ecuadorian embassy, the US would have had no incentive to file for extradition.