Short answer: because she can show off that way.
Long answer: Diana is trained as a warrior who eventually need to fight stronger and greater number of enemies than herself. Just because she could take punches it doesn't mean she should, especially if it could get her off balance. Also don't forget that everyone who was very strong in general was depowered in Justice League to raise the stakes. Both Superman and Wonder Woman was knocked out by mere punches from characters who realistically would break their bones trying to hit them.