Jamieson, Fausset and Brown on Leviticus 22:8

8. dieth of itself--The feelings of nature revolt against such food. It might have been left to the discretion of the Hebrews, who it may be supposed (like the people of all civilized nations) would have abstained from the use of it without any positive interdict. But an express precept was necessary to show them that whatever died naturally or from disease, was prohibited to them by the operation of that law which forbade them the use of any meat with its blood.