There is a lot of circumstantial evidence in that case: the ransom note (and the Ramseys' reaction to that note!), the parents' suspicious behavior in general, their changing elements of the story (from the doors being locked to the contrary, Patsy stating she had read the ransom note first, and then gone to into JB's bedroom, later saying it was the other way round; her contradictory statements re JB's clothing on that night; fibers from PR's red sweater in the paint tray, et., one would get a very long list here.
The Ramseys haven't been brought to justice because
a) the crime scene was completely botched during the initial investigation (the body moved, too many people in the house, etc. so it was difficult to tie specific forensic evidence to a suspect (even Pam Paugh was allowed later to rummage through the house, dressed in a police uniform - incredible -one would think this was a bad police movie, but no, the horror is that this is non-fiction).
and
b) the Boulder DA office was in bed with the Ramsey lawyers, giving the Ramseys kid glove treatment.
Just one typical example: the Ramseys were allowed to give handwriting samples in Pete Hofstrom's private home (to spare them the inconvenience of having to enter a LE building), and while they gave their handwriting examples, Hofstrom actually went out for a walk!!!
One thing is for sure: the Ramsey case is among the most mishandled cases in criminal history. And the Ramseys are profiting from this situation up to the present day.