TOPIC: If 2021 ECO is on 3 domains why does 2021 simulator have the knowledge areas

Since the ECO is on the 3 domains why does the 2021 simulator still have the knowledge areas? Isn't predictive Domain II?

The ECO still covers the topics covered in the knowledge areas: stakeholders, risk, quality, integration etc. They are just spread out differently. Maybe someone will come along with a different answer, but they are still relevant to the exam.
I'm sorry but I don't understand your question about predictive being Domain II. Domain II is Process, but that can cover both iterative and predictive methodologies (Task 8, for example, covers WBS and the backlog as different ways to handle scope).