I can't imagine why they would refuse to do so. Strange.
Their lawyer was on the Clinton News Network just now. I'd take anything he would say with a huge grain of salt, but he argued three points: First, that prematurely evacuating people would have led to deaths, as some of them were in such precarious straits that they could not have handled a bus ride. Second, that they only received a voluntary evacuation order, never a mandatory one. Third, that they called family members and received their okay to keep the patients at the facility (they told family members that if they wanted to pick up their loved ones they'd be accommodated).
Again, I take it all with a huge grain of salt, but I think out of fairness their mouthpiece's arguments should be presented.