I think it may have to do with the issue of foul play. Unlike Riley Strain, when he was found two weeks after going missing, all evidence pointed to misadventure. The identification was made quickly because there was no need to suppress evidence that may have been crucial to a criminal investigation. The autopsy results came in later and supported the conclusion of death by drowning via misadventure.
In this case, there has been an ongoing commentary that Caleb is the victim of foul play, not because there is any actual evidence pointing that way, but a series of circumstances that provided a mystery surrounding his absence which his parents choose to believe. If LE believe there is a possibility of foul play versus misadventure I suspect they want to hold all cards close to their vest because they can't guarantee that the family members will keep quiet about his death and compromise an investigation.