Obviously the Germans have already tried all the other guards, right?
No, they haven't. This pursuit of Demjanjuk is all about politics--the Germans want to atone for their sins by sacrificing a Ukrainian POW they forced into service (if he was indeed at Sobibor), and the Office of Special Investigation at the Justice Department wants to save face over the Ivan the Terrible debacle, where not only did the Israeli Supreme Court overturn Demjanjuk's conviction (and specifically refuse to try him for what the Germans want to try him for), but the Sixth Circuit found that the Justice Department prosecutors had engaged in prosecutorial misconduct and had committed fraud on the court.