Why that cargo door on 1st A350-1000 prototype?

Airbus has installed in the MSN59, the 1^st A350-1000 prototype, the same Emergency escape hatch for test-crew that was installed in the MSN1, the first A350-900 prototype.

Source: Airbus

The main cargo door, manufactured in Korea by KAI wil not be flying on the first set of test-flights of the MSN59. As it occurred in the MSN1 first flight prototype.

Each test flight is operated by a crew of 2 pilots and 3 flight engineers, who monitor the stream of data flowing from a multitude of sensors into a bank of computers installed in the middle of the cabin

Source: Airbus

On all seats is a parachute. If things should go terribly awry and the crew needs to evacuate, a bright-orange railing leads them from the cockpit door to a hatch in the floor above the forward cargo door.

By pulling a lever, the crew can trigger a set of explosive charges that will blow a hole in the right side of the fuselage. They can then leap down a slide, through the hole, and into the air.

Source: Airbus

More details about the emergency scape.

The cargo door could be installed later as it was done beginning of 2015 in the MSN1.