Because it better explains why birds do what they do and live where they do. For most birds, there is a range of environmental temperatures over which the deep-body temperature remains constant. Temperatures above and below the range of thermoneutrality will both result in increased heat production. Thermoneutrality means that the conditions are such that the bird doesn't use energy to either lose or maintain body heat. If the temperature drops too low, heat production cannot be maintained and the bird dies.
Birds die faster due to heat stress than cold stress.
At very low temperatures, the bird shivers - involuntary muscle contractions generate heat production. Birds can behaviorally thermoregulate to some extent to reduce heat loss. The most conspicuous behavior is migration to a warmer climate. Or fluff feathers to trap heat. And Brown Creepers by the dozens huddle together for warmth at night.