It didn't work cross region because no one builds an app expecting latency between their app server code and database. What we'd see on write requests was something like:

1. Query for data from read replica, perhaps for validation (0ms)

2. Do a write to the primary in a different region (20-400ms)

3. Query primary for consistency (20-400ms)

4. More queries against primary for consistency (20-400ms)

5. Maybe another write (20-400ms)

6. repeat

You can actually use our postgres this way if you want! But it breaks for most apps. It is much, much faster to ship the whole HTTP request where it needs to be than the move the database away from an app instance.