The relevant section: https://www.ushmm.org/wlc/en/article.php?ModuleId=10007069
After much debate, 24 defendants were chosen to represent a cross-section of Nazi diplomatic, economic, political, and military leadership.
Adolf Hitler, Heinrich Himmler, and Joseph Goebbels never stood trial, having committed suicide before the end of the war. The IMT decided not to try them posthumously so as not to create the impression that they might still be alive.
In fact, only 21 defendants appeared in court. German industrialist Gustav Krupp was included in the original indictment, but he was elderly and in failing health. It was decided in preliminary hearings to exclude him from the proceedings. Nazi party secretary Martin Bormann was tried and convicted in absentia. Robert Ley committed suicide on the eve of the trial.
Well, in the case of Hitler for example, the concern was justified because even though by the summer of 1945 the Western Allies were very very sure Hitler was dead but couldn't confirm because the Soviets did hold back the evidence they had in order to profit politically from a believe that Hitler was still alive somewhere.
A similar thinking can be seen in the case of Martin Bormann. With him, the Allies were not as certain that he was dead as they were in the case of Hitler and the cases of Himmler and Goebbels (both of whom were confirmed dead for certain by this point). It took a long time for Bormann's remains to be found and identified and for years and years there were rumors that he was still alive and hiding in various places. The guy who bought the forged Hitler diaries for the magazine Stern for example was one of the people who sunk a lot of money in looking for the still-living (as he believed him to be) Bormann.
By trying to avoid even giving off the whiff that people like Hitler were still alive in the form of a trial, the Allies were hoping to avoid to have any remaining Nazis (and there were a lot of remaining Nazis) being still inspired by Hitler and the others to acts of resistance. Having former Nazis believe in the fact that their "Führer" took the "coward's" way out of suicide, the Allies hoped to demoralize them and them to refrain from further resistance to occupation.
Btw. more on the fear of the Allies about a German resistance after the war was over can be read in this post (caught up in our April fools shenanigans but still valid historical info)