I also contact XOM asking about availability the other day. This was the response I received.
Thanks for writing to us. For the moment, all of my Mobil 1 ESP oils have a much smaller market/demand compared to my Mobil 1 oils without the "ESP" identifier. Typically, the greater the demand, the easier it is to justify a wide range of container sizes.
For the moment, we are just on the leading edge of this specific oil's introduction to the market. We're hoping to see enough European vehicles moving to 0W-20 that we can ramp up production, make it easier to find this oil on the shelf, and justify a little more variety in container sizes.
The new Duramax I6 will use this spec oil as well. I read somewhere that the take rate for this motor will be around 10%. That should be 50K plus Silverado's alone per year. Hopefully with that and the Europeans (Porsche, VW, Audi) this will give them a reason to stock it on the shelf at a brick and mortar parts store.