Perhaps at the time that was the simpler route. It's been reported that the US intelligence agencies have been working on the extradition process in 2018, so the fact that this was easily done today doesn't mean it could've been done as easily then.
Edit: HN doesn't let me reply so deep in a thread unless I wait like 20 minutes. It'd have been simpler if he were detained as they'd have had a guarantee of his jurisdiction and plan accordingly. I imagine having him arrested in a country that had allowed CIA extraordinary rendition would've been better for them than having him protected in Ecuadorian embassy.
[1]: https://www.smh.com.au/world/europe/wikileaks-founder-julian... [2]: https://www.theguardian.com/media/2018/nov/16/julian-assange...