because the family's various re-tellings of what happened that morning don't make sense with each other (note - i'm not blaming them for anything, i'm just stating a fact). you can be an apologist for using words that aren't synonyms, like "dragged" vs "walked" but that's a stretch imo. the timelines don't match up, the story doesn't make any sense, and the retellings i've read and heard from the horse's mouth lack the rich detail you'd get from the average person retelling an important story.
it's the difference between "i went to the market this morning and purchased a bag of red delicious apples because they were on sale" and "i bought some fruit". none of them have given a publicly available run-down of that morning that wasn't mostly one word answers.
please note - i've said from the beginning that i don't think they have anything to do with this, however i do not think the story they are telling is the entire story. we know le asked them not to release info about the blood, we know le didn't want the public to know about the phone being found, and i firmly believe there is something else going on that they're not allowed to talk about.
possible theories - 1) as others have said, the family has a good idea who the perp is but he has an alibi, 2) there's an indication holly is alive but in danger, 3) cb saw more than he is allowed to say, or perhaps heard snippets of the conversation.