The state of TN does have the death penalty, with 86 prisoners currently on death row. Of those 86, 40 of them were convicted in west TN.
IMO killers do not hide bodies to be used to negotiate a better sentence later, 99% of crimes are committed in the mindset that the criminal will not be caught. The "plan" is to commit the crime AND get away with it, every action they take should be looked at from that point of view.
I think the bottom line there is that it is hard to prove John Doe murdered Jane Doe if you cannot first prove Jane Doe was murdered. Also, the CSI effect plays a role in this because of forensic advancements over the last couple of decades.
Also, a victim's family cannot seek the death penalty or negotiate using it, it is completely up to the prosecution.