For the same reason that you cannot move citrus from one quarantine area in Alabama to a non-quarantine area in Alabama.  Both states are recognized as citrus producing states and can only import citrus trees that have been grown in USDA-APHIS approved structures from certified clean stock. Producing citrus trees for interstate commerce is very expensive and citrus trees grown in the majority of the nurseries found in these states, do not meet the criteria for interstate commerce, therefore it is illegal to move or transport citrus from one citrus producing state to another citrus producing state if that nursery does not meet the strict USDA Citrus Nursery Stock Protocol.  There is no distinction between a commercial entity transporting the trees or a homeowner.