The reason to avoid trying him in the UK is that the penalties are lesser than in the US and the fact that the crime was committed in the US (allegedly). Also the fact that all British hackers seem to develop Asperger's with their mothers crying on the news that their baby will die if they go to the US makes it hard to take the UK judicial system as anything other than extremely lenient.
The US judicial system is much more adversarial in nature not to mention expensive with harsher penalties and less likely mitigation of sentencing than then UK system but it does have some checks and balances and frequently the innocent do go free. But not always but then prisons are every where only have innocent people in them if you talk to the prisoners.