Why would some life forms change and others stay the same for millions of years?
Change only occurs if there is selection pressure on the particular population. Some species are very similar to they way their ancestors looked millennia ago. Others have changed significantly.
Species that live in stable environments such as the depths of the oceans change less. That is why coelacanths show less change.
For the fourth time - What is the most significant difference between the human and primate knee joints?