The BLM anthropologist has deemed that the site of interest is not a site of antiquity. That is, it is newer that 50 years old. That rules out pioneers, native Americans or even WWII internment escapees. Still, a 50 year range leaves a lot of room to not be Susan. The police seem to think that it is much newer than that however.
Secondly, everyone is hung up on what Josh did that night they went camping. Don't forget that the police confiscated his vehicle and he had to rent a vehicle for several days. During that time period he put several hundred unaccounted for miles on the rental. Enough to easily drive back out to the site 1 or 2 more times to clean up the site and erase traces of human, or specifically his, involvement.
Driving back to the place where you dumped your wife's body in a rental after the police have seized your car to process for evidence would be an unspeakably stupid thing to do. Driving to Ely to try to find her shacked up with a druggie boyfriend would be stupid, too. Driving anywhere while police are swarming over your home and car would be just about the only thing you could do.
Being unwilling to cooperate with the police, who promise to use everything you say against you is not stupid, and it's not a real reason for the police to consider him the prime suspect. There are other, more obvious reasons anybody can surmise from the most casual acquaintance with the situation, which is why most folks think he did it.
If he did it, and the cops still can't find enough proof to make the arrest, he'd have to be smarter than most of us. Which he isn't.
The cops haven't impressed me with being very smart either, but if they have located Susan's remains I'll be changing my impressions pretty quick.