While some Nazi Defendants were put to death and others served prison sentences and eventually got out of Spandau Prison, Hess, who served much less of a role in the Nazi war machine, was sentenced to life.
This answer by /u/kieslowskifan to the question "It seems that all Nazis sentenced to life in prison after the war were released by the 1960s, with the sole exception of Rudolf Hess, who was still incarcerated when he died in 1987. What reasons were given to not release Hess?" should answer your question.