Why did conformers not evolve to become regulators?

Organisms which cannot maintain their constant internal body temperature and osmotic balance are known as conformers.

Organisms which maintain their constant body temperature and osmotic balance are known as regulators.

Heat loss or gain is a function of the ratio between surface area and volume. Small animals have a large surface area to volume ratio. Thus a lot of energy has to be spent to regulate temperature. To conserve energy conformers have not evolved into regulators.