Why Spanish treasure fleets had to land at Cadiz

Why did Spain require a treasure fleet to land only at Cadiz?

Cadiz was considered the one port secure enough, and controlled closely enough by the monarchy, to prevent part of the cargo disappearing before it reached the state.

A treasure fleet carried wealth large enough to influence national war policy. The monarchy therefore wanted its cargo discharged at a port it could supervise directly. Cadiz offered the required combination of physical security and political control. The rule reduced internal leakage, but it also made the fleet's destination predictable to British intelligence and interception forces.