It has been said that there are other deaths that would still put the hospital over their usual amount of deaths in that unit in that year long period.
I am thinking that even though LL was MAYBE on the floor for those other uncharged deaths, they may not have been clear cut cases that the prosecution could win. For example, they may have included babies with serious underlying medical issues, or babies who were fragile and underweight or severely premature, or there may not have been solid evidence placing LL in that child's room preceding the collapse.
If any of those circumstances existed, they may have decided against bringing charges, so it wouldn't muddy up the cases where they allegedly had more circumstantial evidence.