Here's why:

* Warm-blooded (homeothermic) animals maintain a stable internal body temperature regardless of the external environment. They do this through internal mechanisms like shivering or sweating.

* Cold-blooded (poikilothermic) animals rely on external sources like the sun to regulate their body temperature. Their internal temperature fluctuates with the surrounding environment.

Protozoans are single-celled organisms and do not have the complex physiological mechanisms required for either warm-bloodedness or cold-bloodedness. They are ectothermic, meaning their internal temperature is determined by their environment.

In simpler terms:

* Warm-blooded animals: Like humans, they have their own internal "heater."

* Cold-blooded animals: Like lizards, they rely on the sun to warm up.

* Protozoans: They're like tiny, single-celled beings, too small to have their own internal temperature control.