What’s the decision rational behind putting the stepper on the bottom of the page?

Taking users through the process. Emphasizing what can’t (shouldn’t) be missed in the flow.

Any bottom over top though? Naturally we read things top down

That’s what you were asking! The rationale: you interact with the critical piece of the feature (workspace) that requires more thought process and when you’re all set, you intuitively engage with the stepper, signaling for what comes next - “everything I need to configure is up”