Because whether he is charged or not, found innocent our guilty if charged or let go, it's certain that he will then be extradited to the USA where he has already been found guilty of crimes that actually aren't crimes due to his nationality and status as a journalist.
Sounds like pretty extenuating circumstances for a Swedish investigator to go to Britain to me, unless there's an actual law that says he MUST be questioned in Sweden
Why shouldn't it happen in Sweden? If they have the legal power to arrest him and question him, why should they bend over backwards and do it his way instead?