Why diplomacy still crossed the Atlantic by ship

Why did Britain still rely on ships for transatlantic diplomatic dispatches during the Trent crisis?

The early cable network was not yet dependable, so physical government communications still had to travel by ship.

The exchange places the Trent crisis in a communications network that was still incomplete and unreliable across the Atlantic. The available undersea-cable link was not yet a dependable substitute for ship-borne messages. Diplomatic instructions and replies therefore continued to cross the ocean physically, magnifying the practical and sovereign importance of Royal Mail steamers. Further cable-laying work by SS Great Eastern was part of the infrastructure needed to improve the connection. This dependence explains why interference with one mail ship could slow diplomacy as well as insult the flag.