Why do some professors have M.D. as well as/instead of a PhD?
There are two main tracks of medicine: clinical (the "standard" medical practice), and research. Typically, doctors who work in the latter tend to pursue combined MD-PhD programs, and consequently have degrees in both a traditional scientific research discipline as well as medicine.  

It probably was easier in the past to make such a switch with "just" a medical degree; however, the increased specialization in the field makes it harder for a medical doctor to switch over to a research track without the increased training (unless he or she is working on the "clinical" side of therapeutic treatments designed in research programs).