Until we knew exactly what we were doing! Apart from the moral/ethical issue of making Mars into the Earth, we have no idea of which of many terraforming techniques will work. We do not know if there will be 'restorative forces' in the martian atmosphere and crust that would defeat any of our efforts. Since the experiment will take billions of dollars and centuries to complete, we need to fully understand Mars before investing the time and money. Hell, we can't even build a self-sustaining ecology to support humans on Earth in a closed habitat, and that's after studying biological systems for centuries!