Actually, Darwin never said man evolved from apes. He did say that man and apes had a common ancestor someplace very far back in history (about a million years or so).
Evolution happened because of changing climate and conditions in certain areas. Forests and jungles changed to grassland in many areas, giving any primates that walked an advantage. It gets complicated from there and all the pieces of the puzzle are not in place, but that, at least is the start.
Apes and monkeys did not evolve probably because there were still jungles and trees where they were.