Well, they already found the body...that's why the guy confessed. He could have told the cops years ago if he was concerned with avoiding the death penalty.
Benton County isn't exactly rolling in cash right now. Capital punishment cases are expensive. I can see why the DA wouldn't mind taking a plea deal with no trial and no possibility that the guy ever gets out of prison. Since he's currently in prison in New Mexico and won't be released for many years, I can see why the family would ant the DA to make a deal, since otherwise, they'd have to wait for extradition, which can be time consuming followed by a long trial. That would make their nightmare come back all over again and last for many, many years. At least it's over for them.