Because trying plaintiffs in their absence is difficult under German rules of criminal procedure (Sec. 230 Para. 1 StPO[1]). Exception exists only if the plaintiff leaves a trial after their initial statement, does not appear for an adjourned date(Sec. 231 StPO[2]) or if the plaintiff intentionally caused their absence by rendering themself unfit for trial (Sec. 231a StPO[3]).
According to the court's press statement[4] (in German) they decided that the plaintiff is not fit for trial but did not culpably cause this unfitness. The court decided to separate the trial and postponing it until the the plaintiff recovers. They explicitly did so to no longer postpone the trial against the other plaintiffs indefinitely.
Criminal law in Sweden allows both - if no specific reason exist for a psych evaluation of the accused, the person is assumed sane. That's the basic assumption.
If the person doesn't show up, a trial is often postponed once or so, but ultimately, if the accused doesn't show up, the trial can be held anyway if it is considered to be performed satisfactorily despite the absence.
I'm certain regarding in absentia, and doubt the "assume sane, then modify the sentence afterwards if assumption turns out to be wrong" very very much.
To be more precise: it's possible to do a trial in absentia, but only if the maximum sentence is a financial penalty, reprimand, loss of driver's license and so on. Paragraph 232 Criminal Procedure Code (StPO).
