Because she died 8 hours later from dehydration. 8 hours is plenty of time to treat dehydration.
"tragedy" isn't the first word that springs to my mind when an organisation tasked with the care of a minor misses the fact that she's dehydrated when only a modicum of attention was required to spot it and only bothers to do something useful when she's on the verge of death.
Yes, she might have died even if border control staff had immediately recognised her symptoms the moment she entered their care. But then 8 hours passed, during which time it's perfectly possible that someone could deteriorate from perfectly healthy to dehydrated even when they haven't been walking through a desert.
I'd bet that any parent in a first-world country whose child dies of dehydration while in their care would be facing criminal charges. The same should happen to whoever is running that facility.
We have a fiduciary relationship with our government and its agencies. I well understand that Trump has threatened or frankly violated that trust on multiple occasions, particularly when it comes to humane treatment of immigrants and asylum seekers and families. This event brings that wound to the surface. We did not create that wound, but it is ours to bear. If we refuse by levying premature blame, even if ever so slightly, we close ourselves off to learning all that can be learned from this death. If we wish to heal our wounds through creation of stronger institutions which prevent another Trump from happening or at least limit the damage done, we must allow ourselves to learn from atrocity which goes far beyond identification of villainy.
There exists many unknowns in this case. For instance, there is no autopsy. The medical records have not been released. The cause of death cited includes sepsis which is severe infection. We don't know what if any underlying conditions she had. We don't know her condition at presentation. Perhaps she appeared as a sleeping child and family voiced no complaints. Perhaps she was medically evaluated initially and was in a much more stable state. Bacterial meningitis can take a mildly ill-appearing child to death in 8 hours for example. Anyone who does critical care work with children knows they compensate extremely well until they can't any longer and crash suddenly. We don't know what care was attempted at the center. It is reasonable that she could not take PO and would require an interosseous line to get fluids in her. She might have had what most people would think of good care and experienced refeeding syndrome hastening her decompensation. We don't know the validity of the reports that she hadn't eaten or drank in days. We don't know the medical acumen of the people who interacted with her at the facility or the treatment resources they had available. We don't know if, e.g., there was another medical emergency going on at the same time which occupied staff. Or appropriate staffing wasn't available due to a car accident, illness, no show, whatever. In other words while it is quite reasonable to suspect neglect or malice could be involved, we don't actually know shit.