The crimes happened outside in the elements/>>>wind, water, time elapsed overnight, no surfaces for fingerprints, etc.
Most of the DNA in crimes inside homes or vehicles are found on victims bodies or on doorknobs, or sinks, or cups or utensils, etc. Hard surfaces that hold the DNA to be discovered later.
The problem with this case is there was no regular type of SA, and no hard protected surfaces to hold DNA or show fingerprints or touch DNA. Then you have the outdoor elements that destroy what little touch DNA wood have been left.