Why do you think that only single-celled organisms live in the hadalpelagic zone? give a detailed answer.

No. Not only single-celled organisms live in the hadalpelagic zone

Explanation:

The deepest zone of the sea is called  “The Trenches” or Hadalpelagic Zone. This zone is described as beginning at approximately 19,000 feet and extending to the ocean floor. At this depth, there is no visible light, so there are no plants, resulting in very little food being available to sustain animal life.

Without single-celled organisms, there are many more organisms in this zone. They are not single-celled. Some of them are:

These all are multi-cellular.

it cools slower in the earth meaning the crystals have more time to form than lava on the surface