Why did modern ape not evolve?

When things evolve, the whole species doesn't change. Theres one member of the species that is born different (slightly) and if it does well then it passes on its genes. The old species doesn't go away unless for some reason it stops doing well and dies out (by "doing well" I mean finding enough food and reproducing.) So to answer your question in a more straight forward way, they DID evolve because they aren't the same as prehistoric apes. They just didn't evolve in the same way we did.