I cannot find why a man can marry his niece but a woman cannot marry her nephew. I know it’s midderabbanan but can find no logic for the difference.

The prohibition for a man to marry his aunt (and hence a woman to marry her nephew) is De'oraita (Vayikra 18;12-14 has all variations of aunt) while there is no verse in the Torah prohibitting a niece. In fact, Sarah was Avraham's niece.