First of all a massive head blow would cause massive bleeding in the brain, and if you read the autopsy, Meyers notes that the purple striations throughout the entire right hemisphere would indicate that is exactly what happened. Secondly, the parietal lobes were bruised and there were petechial hemorhaging in the eyes. Any neurologist that looked at a catscan of that brain injury would know that child was being shook - thirdly, there are at least three visible furrows on her neck and a bruise on the front of the neck not just on the back where the knot of the furrow was which looks remarkably like marks from where knuckles would have pushed into her neck.
In my opinion, someone took that little girl by her shirt collar, started shaking her, she reared her head back and hit it on something - you would have heard the head hit whatever it was - with an injury like that she would have immediately lapsed into unconsciousness and appeared not to be breathing. She probably was paralyzed at least completely on one side if not both.
So, the question is, how would anyone explain all those injuries, especially if they thought the child was dead? Unless you're completely deaf, you would have heard that head hit something. And then you have to explain the neck furrows, where the child was when the injury happened, and how the injury occurred. When some posters offer the theory that it was an accident, they don't mean she fell and hurt herself and then the parents covered it up, they mean the death was not intended - if it was involuntary manslaughter - no one deliberately set out to kill the child. However, even if it was not intended, it is still a crime, and though we know now that the Boulder DA never prosecuted if they could help it, the person who did this would not have known it at the time, and even if the person did know, there is still societal opprobriation and rejection that the person would have to face. For some people even that is too much to face and have to accept - what would family say, what would friends say, what would co-workers think?
Many people in that kind of situation would panic and try to cover it up, (and they have tried) that's just human nature to want approval from society and they're willing to risk quite alot in order to get it. The problem is that when were in that state of panic we're not thinking of long range consequences and because we're not thinking clearly we do really stupid things.