Simple negligence is the absence of due care; that is, an act or omission of a person who is under a duty to use due care which exhibits a lack of that degree of care of the safety of others which a reasonably careful person would have exercised under the same or similar circumstances. Simple negligence is a lesser degree of carelessness than culpable (willful) negligence."
And this is why they dropped the case. They had no evidence that her forgetting exhibits a "lack of that degree of care of the safety of others". Reasonable people forget important things with tragic consequences all the damn time. They have to prove that it was "lack of care" rather than human error attributable to external factors like tiredness, distraction and habituated routines.
That's tough to prove when the parent is a concientious, hard working, involved parent with tons of character witnesses as opposed to the partying, gambling, shopping type of parent, where it's obvious the parent frequently placed their gratification ahead of the child's needs and you have a line around the block of people willing to comment on what a useless scut the parent was.
The fine line between human error and "lack of care of the safety of others" is properly decided by the prosecutor in determining whether to pursue the case, not by the judge or jury. The jury's job is to affirm (or deny) whether the state has proved it's case if the prosecutor determines there is sufficient evidence to prove negligence.
There is a growing body of literature on this phenom from both the legal and the sociological and medical povs.
To quote a freeper on a previous case like this
“How are you going to fix this? Tell people not to forget?”