Why did not Poikilothermic Animals become Homeothermic?

Some animals maintain temperature homeostasis in the body due to external sources of energy (poikilothermy), others due to the energy of food consumption (homeothermy). Homeothermy (“warm bloodedness”) is maintained endothermically, i.e., by-product of metabolic heat. In poikilothermic animals (“cold bloodiness”), in contrast to homeothermic animals, body temperature passively follows the change of the environment temperature. It is known that homeothermy takes place when living beings has a high level of cellular metabolism. However, the cause of this high metabolic rate remains a source of controversy.