Plug connectors use plugs and sockets, so wires first have to be pressed, welded or soldered together before they can be fed into the plug connection. This method is especially practical if connections have to be removed regularly. This then produces multiple additional contacts that increase resistance and cause voltage loss. For the sake of energy efficiency, then, the live main line should not be used. Contact points are also high-risk. If these are arranged in a row, one contact point failing will cause all the following points to fail too.
With a flat wire, the live main line is not interrupted and is also not affected by a contact point failing. The current flow is consistently optimal.