Often when someone gets a doctorate in nursing, they are planning to work in academia and teach, not remain in the clinic with patients. In a clinical setting, there's no reason for them to use their title when being introduced to a patient, especially if it will cause confusion.
Technically, any Ph.D. degree is a higher degree than an M.D., but of course, that's irrelevant in a patient care setting where specialization of training and knowledge is more important than how much education you've received. Afterall, someone with a Ph.D. in physics isn't going to be able to walk into a hospital and offer medical care either.