Because the theory of evolution postulates that things evolve from simple to complex – not from complex to simple. Therefore an ape could not come from man because an ape is less complex than a human.

That humans evolved from apes is a common misconception. Evolution seems to be the most misunderstood theory of science... Humans shared a common ancestor with apes, and this group of ancestors (in the case of chimpanzees, they lived 6 million years ago), split into two geographical areas. One part of this group found itself isolated in different conditions and became us, the others KEPT EVOLVING, just in a different direction, but also they made progress. Evolution says that all life evolved from an ancestor organism living 3.5 billion years ago. This organism was some form of bacteria. So yes, it goes from simple to more complex, but all life forms now are equally evolved. All of them have ancestors going back to this first species of bacteria (or molecules at the beginning, but it's hard to call a molecule 'ancestor). Imagine it like a huge tree of life, and as time passes, the tree branches many times. This means all life on earth is equally well adapted to it's environment. Look at bats, they use extremely sophisticated radar locations for example. Humans are superior in intelligence and consciousness, so we are indeed special, but every animal is extremely good at something. Otherwise they woulda gone extinct.