Another term for dead meat is "roadkill" or animals that were killed by predators. The reasoning behind it, is you have no idea how long it has been there (how long bacteria has been growing), what kinds of diseases it may have received from a predatory animal and it was not drained of blood in the manner prescribed by Allah (swt). It is called "carrion" and is likened to eating scavenger animals. Scavenger type animals eat anything, and what they eat is not broken down in the body, so in essence you are eating whatever gunk they ate. Yuck.