Well, for one, nurses (except for nurse practitioners and anesthesia nurses) don't make medical diagnoses (at least not formally). Another reason is because--and I hate to sound like a misogynist--nursing has historically been and still is a profession dominated by women. I am a registered nurse (and a guy) who works in a cardiothoracic intensive care unit. Though I disagree how it is, I often see how a male physician will act and respond considerably different to me as opposed to a woman nurse. Not all physicians are not like this--a good majority aren' like this--but it should not an issue at all. Another reason is the entry level education for a registered nurse: an associate degree which can be obtained at a technical/vocational college. Medical doctors and nurse practitioners require intense post-undergraduate work and residency. There are other reasons, for sure, but these are a few that come to my mind.