Because he had nothing to do with it.
The




was found on the computer at the mothers house, at which the mother, an adult male and a teenaged male resided. I think you should be able figure out who's




collection it probably was, doesn't take a rocket scientist.
There were 100k+ images of




found by the sheriff's department on that computer. None of that was CP though, the CP was found subsequently as deleted files by the FBI. They didn't say how much was there, but given the size of the collection and the probability that it had been downloaded from public sources, not commercial, the chances are good that CP could have been accidently downloaded and then deleted. That would not be criminal beyond copyright infringement.
No CP was found in BD's house or SA's possesion. Some




was found on a CD and on his phone, but that was legal




. He did not have a computer in the house, which is a piece of equipment someone who collects




would definitely have.
When he did use computers, it was apparently mostly on the computer at the grandmother's house. Which makes sense because it was a lot closer, and on the way to work. No




was found on that computer, except for an email image that appeared similar to some images found on the mother's computer. But, that image apparently was not illegal, and it presumably was something that had been sent to him. Again, nothing illegal.
There is no evidence at all that he has an obsession with even




, let alone child




.
So what exactly should they charge him with, given that all the evidence suggests that he had nothing to do with the CP?