This probably is more of a legal question than a history question but why was there never a retrial after they went on tv talking about it. Does that not count as new evidence allowing for a retrial?
You’re right, it is more legal than historical. The killers were acquitted by the jury and therefore protected from double-jeopardy by the 5th Amendment so a criminal retrial was not possible. The same holds true for other famous killers who later confessed like OJ Simpson. Double-jeopardy does not protect against civil trials which is why Simpson was successfully sued for wrongful death.