He was extradited to the US because there is an extradition treaty that gives the US the power to extradite people from the UK for meeting a reasonable suspicion test that they committed a crime in the US that is a crime in both countries and carries a sentence of at least a year.
The treaty is reciprocal. The UK has an equivalent right with respect to people in the US who committed a crime in the UK who meet a probable cause test. An independent review concluded that the two tests used in the two countries are worded differently but effectively equivalent.
As far as I know, this is the first UK extradition request to be denied by the USA, while the UK has denied at least 10 extradition requests to the USA.
The UK won't extradite for any crime that carries the death penalty, because they believe the death penalty is inhumane. I believe that accounts for all of their denials.
That is such a shameful abuse of diplomatic immunity. We give out diplomatic immunity because we recognize that diplomats have to live in a country they aren't loyal to and so want make them feel safe that they wont be charged with Treason or Espionage crimes. It's not for when you kill someone while driving recklessly...
The treaty is reciprocal. The UK has an equivalent right with respect to people in the US who committed a crime in the UK who meet a probable cause test. An independent review concluded that the two tests used in the two countries are worded differently but effectively equivalent.