I think it has mostly to do with the fact that being a nurse used to be mostly reserved for females. Most men won't consider it to be a potential job for them to do because of this. From a young age, when you start learning about occupations, you were most probably shown a picture of a female nurse and that's the concept that sticks. It's the same as people buying pink clothing for girls and blue for boys, as that conforms to the general norm of our societies. It doesn't have any real relation other than that which we created.