It became a much bigger deal once he fled first Sweden, and then British jurisdiction. That set up it as a test of the powers of the legal systems of those countries.

If you allow someone to escape prosecution simply by raising the costs/creating inconvenience, you all but guarantee others to try to do the same, and/or to lose faith in the equality of justice.

Regarding the common conspiracy theories: always remember that he was arrested, then freed on bail, in both countries. If they had wanted to extradite him to Guantanamo, they would've done it long ago.