Just to be clear: Android is NOT becoming closed source! Google remains committed to releasing Android source code (during monthly/quarterly releases, etc.) , BUT you won't be able to scour the AOSP Gerrit for source code changes like you could before.
I am not sure, there was nothing stopping them from taking it fully closed now in one step. It is more likely making it open only after release is a measure to avoid all the community input spaff during development. So long and thanks for all the feedback.