Given the politicized nature of the crimes he's charged with in the US, neither country was likely to extradite him unless they had a politically safe reason to arrest him in the first place.
It's for an alleged "conspiracy to commit computer intrusion" with Chelsea Manning where the US claims he helped Manning get access to a passphrase so they could get some more documents[1]. We found out -- though it was criminally under-reported -- in 2018 that a case had been filed by the DOJ[2], but it's now public.
