al theory surrounding cravings is that the body craves foods that are high in certain nutrients it is deficient in. Therefore a pregnant woman who thinks the soil in the garden looks tasty is perhaps deficient in phosphorus while the lady on My Strange Addiction that
eats toilet paper is probably just weird.
Now, as you may not know the brain is made up largely of fat, due to myelin, a fatty substance that surrounds nerve cells and insulates them to prevent cross talk with other cells. Fat is important in basic body functions, especially the functions of a zombie. Fat stores energy (which is important because we speculate that zombies burn a lot of energy very quickly), helps maintain body temperature, and cells need it to function (and since a zombie's cells can not undergo division it is imperative that they protect the ones they have).
Flesh is a really creepy sounding name for the skin, fat, and muscles of a human. Muscles are made up of protein which, as most people know, is a source of energy. The Zombie must consume flesh for energy. Things such as Red Bull or Monster simply cannot compete with the pure intense energy provided by Normals.
DISCLAIMER: This is NOT an argument for cannibalism. College students, overworked coal miners and old people should NOT revert to eating their friends and family to keep going. A Zombie is more adept at burning human flesh than a Normal. Cannibalism is not socially acceptable.