As you may know in three major religions human is forbidden to eat all kind of animals and it has been limited to a rule of some sort.
Religious people may not wonder why and non-religious people may criticize it.
Here is why:
1. All the animals that allowed for human to be eaten are fed with vegetables. All predators or those that naturally may live or eat in filthy area is forbidden. Swine is an example. Even though farmers may keep them in a clean environment but this animal naturally eats and lives in dirty area. So the first reason is that these animal are not affected by blood or meat of other animals
2. If you look at nature, each animal is only feeding on a certain range of food. You can't find an animal that eats everything on earth! This is to keep the nature in its balanced cycle and is crucial for ecosystem. Other than that, all the creators would be now extinct. Now imagine human were also allowed to eat anything to begin with, There would be nothing on the earth! There are countries in the world that people there eat anything from dogs to snakes and from whales to worms. This is a real threat to nature, in particular if this was extended to the whole world.