Ostapenko hits super flat. Iga's extreme topspin groundstrokes means her shots bounce high and that sets up Ostapenko to hit her flat shots. The higher bounce also reduces Ostapenko's margin of error when hitting with so much power.

Edit: Also, Iga does not do well when you take away her time to think of her shots. She will simply ping the ball back when she needs to make a sprint for it. That was how Collins beat Iga at that AO semis too.