Asking why Hillary shouldn't be president and why we can't have a woman president are two completely different questions.
There's no reason why we can't have a woman president. To say otherwise would be sexist.
However, to claim that this premise therefore means Hillary should be president is incorrect. In fact, it's far more sexist to imply that if womankind is to have a hope of the presidency, that it can produce a candidate of no higher competency and moral fiber than Hillary.