As I understand it, a divorced Catholic who remarries is involved in an ongoing sin. The fact that this sin cannot be absolved has nothing to do with the relative severity of the sin. It has everything to do with the fact that continuing to practice the sin is indisputable evidence that the sinner has not indeed repented and "turned around," as is required for absolution.
In a murder case, the equivalent would be the murderer who announced in the confessional his intention to continue to kill. Such a person is by definition not truly penitent and thus cannot be absolved of sin.
The priest must believe that the sinner is penitent before absolving him.