My understanding (based on reporting [1]) is that the Obama administration was very much on the fence about prosecuting Assange, largely because they were concerned about political blowback and precedent that it might set for prosecuting journalists. The added blowback from using the Swedish rape charge as a "backdoor" extradition would have made the case politically even more charged. So there was a view that, while extradition was possible, it was so politically fraught that Assange had a pretty good chance of walking away from it. This was also a time when Wikileaks/Assange was relatively more popular, since they weren't perceived as taking a partisan interest in an election outcome [2].

Then Donald Trump got elected and Assange was doomed.

[1] https://apnews.com/3d9c190f66cc4e5b8669bcc0b6c1eff9 [2] https://theintercept.com/2018/02/14/julian-assange-wikileaks...