They all still exist because they simply had better body structures to stay in the forest. Why change a perfect body structure tailored to your environment? Its also why crocodiles have barely changed. Not every animal in a species evolves. Only the ones who were able to diverge and give new genes. The others still have survived long enough to be part of their environment so why not keep a good thing going? Overpopulation could drive some animals in a species to new places and make new adaptions creating new species, but still leaving the original one intact.