The pool of participants is much narrower in scope - people that have specifically spent years training to be that profession. Not people who started out doing it as a hobby or a quiet thing on the side.

I think the OP was asking how requiring a degree is relevant to the number of men in those professions. When compared to women, men generally have equal or better access to higher education, so it would seem that the dearth of men in nursing and teaching wouldn't be related to those professions requiring a degree.