Exactly the reason.

When a person is sent to death row they cannot just accept it and let things take their course, instead they are required (yes required) to make no less than 3 appeals to the verdict in the hopes to be commuted to life sentence or whatever they can get.

The average for the appeal process is upwards of 8 years and is very expensive because 99% of the time the lawyers used are paid for by the state in which the person was convicted in.

This process was enacted years ago in the hopes that no one person would be sentenced to death wrongly and would have a proper chance at proving their innocence or the lesser degree of the crime. It's kind of a double edged sword since by enacting this process they are pointing out that the judicial process is or can be faulty (we all knew this anyways) and that people will lie to cover the truth even under oath (we also knew this)