I bet it's because they system is very hierarchical. Once a doctor says something the nurse can't really override it.
The NP could still have done it, but it would have been against two different doctors and the organization who employs her practices. The sad thing is that this probably happens often, this is just the first time someone sued over the death.
edit to add: possibly the NP went up the chain of command precisely because the doctors there wanted veto power on hospital admissions. they didn't want pesky underlings getting in the way of whatever motivated their decisions.
Once two doctors say something.