Here's why:
* They lack internal temperature regulation: Unlike warm-blooded (endothermic) animals like mammals and birds, viperfish don't produce their own body heat through metabolism.
* Their temperature fluctuates with the environment: Their body temperature changes with the surrounding water temperature.
* They are adapted to deep-sea conditions: Deep-sea environments are very cold, and viperfish have evolved to thrive in these temperatures.
While viperfish have a special adaptation called counter-current heat exchange in their muscles that helps conserve some heat, they are still fundamentally cold-blooded.