Deuteronomy 14:21
Ye shall not eat of any thing that dieth of itself: thou shalt give it unto the stranger that is in thy gates, that he may eat it; or thou mayest sell it unto an alien: for thou art an holy people unto the LORD thy God. Thou shalt not seethe a kid in his mother’s milk.
Animals that died by themselves were forbidden as food because they were not fit to be eaten. The restriction applied to Israel while living in Canaan, where the permitted flesh was confined to animals in sound and healthy condition. By allowing only such animals, the dietary rules reduced the injurious consequences that follow the use of animal food.