My opinions only, no facts here:
It seems probable that the various involved authorities have had at least a few, if not several formal interviews with each of the three arrested main suspects. When the authorities possess incontrovertible evidence (such as relevant DNA, relevant fingerprints, etc.), they always lay that evidence out in the interview room to elicit a confession on-the-spot. There is nothing coy or mysterious about the strategy or motive of the cops in the 'room'; they want a confession. Since none of the three suspects has plead guilty yet, even when now facing the death penalty, I would guess that incontrovertible evidence was NOT presented to these suspects during their respective interviews (they still believe that they can win in court). Looking back in time, I would presume that fellow Tennessean Paul House also refused to plead guilty, for his own particular reasons- but without immediate success! The Gussie Vann case is also interesting for Tennessee crime fans.
As for terabytes of evidence; sometimes a very rich man opens his giant four-car garage door to his mansion and all that comes out is his three-year old son on a tricycle.
When the Prosecution's evidence is made semi-public (hopefully through Discovery by the Prosecution to the Defense); only then will I know what the heck is actually going on here. In the meantime, I keep researching this fascinating case from the very beginning, out of bad? habits. And that includes completing my Part IV Chapter in the Holly Bobo case- The Cell Phone Evidence.
I honestly am flabbergasted that none of the 3 arrested suspects has plead guilty to lesser charges and ratted on their associates by this late point in time. It makes me scratch my head in wonder and confusion.
Sleuth On!