Because the authorities probably requested/advised/ordered them to evaculate. With justification - you don't want to be in a beachfront home when there's a hurricane on it's way with a possible 8 foot storm surge.
Here in Virginia, when they issue evacuation, the difference between “mandatory” and “voluntary” is if it’s mandatory they say “you can stay, but if you get in trouble we ain’t gonna come get you.” They can’t make you leave, but you’re on your own.