Why not? It seems a fair number of powerful people in the US would love to extradite him.
In other words: The US is very successful at getting "ordinary criminals" extradited from the UK, but not so much when it comes to thornier cases like or example McKinnon's. Maybe he eventually gets extradited, but it might still drag on for ages.
Meanwhile, Sweden has a history of having had its police drive people who were in their country seeking political asylum to the airport, handing them over to masked CIA agents and waiting while said CIA agents stripped the asylum seekers naked, drugged, handcuffed and chained them, and threw them on a plane to send them back to the Mubarak regime where they were tortured - Sweden has admitted as much, and paid restitution to at least one victim for torture as a direct result of Swedish actions in blatant contradiction of Swedish law.
There was a massive outrage. Shortly afterwards, Swedish military intelligence determined that US intelligence agencies where still carrying out illegal rendition flights via Swedish airports with full knowing compliance from Swedish airport staff who apparently didn't get the memo... Or who got the wrong memo. Or something.
I know which of the two places I would take my chances if I'd pissed off the US government.