Other than people shooting at those trying to bring it in. Or did you mean why it wasn't in a designated shelter area to begin with? The answer is that the 'Dome was a shelter of last resort, intended only to shelter from the wind and rain of the storm itself, not a refuge from the rising waters of a levee break. When the levees broke and the water started rising, they still had those buses, some of them just over a mile away, that could have been used to haul people out of there... assuming they didn't shoot at the buses.
It wasn't designated a shelter. It was a refuge of last resort.
A shelter is is to have food, water, medical people and security.