But anyone deploying a critical application to AWS makes a point of cross-region data replication. Heroku have long known that they lose potential customers to, say, Engine Yard as a result of only hosting at US-East.

One can only conclude that this is a clear business decision on their part. I can hardly believe that Heroku's engineers are incapable of it. Indeed I would be very surprised to learn that they haven't brought up an instance of their platform at, say, US-West, for testing or proof-of-concept purposes.

Of course, productising that is a different matter. Extending the control plane, front end, and pricing/billing systems might have considerable associated project cost. Perhaps they have concluded that the costs outweigh the additional revenue. Or, just haven't got around to it yet.