Perhaps not so surprising given that Amtrak are giving up on double deck coaches and are planning to replace Superliners with single level coaches (and, sadly, it appears that they won't be taking advantage of this to re-introduce dome cars).
I haven't been following it in detail but I've seen it suggested that accessibility requirements are partly to blame.
Trains magazine's correspondents cited two principal factors: the desire to have a single long distance fleet type capable of operating in the east as well as on transcontinental trains; and as you say, the concerns expressed by potential builders about Amtrak's accessibility requirements, potentially needing powered lifts in every bilevel vehicle.
When I read about these issues I did wonder how 1950s BR DMUs managed, large glazed fronts, simpler glass technology of the era, and presumably similar hazards at similar speeds.
As you may recall, the attitude to risk in the 1950s was very different: railway carriages had manual doors without any locking, many of them were full of cigarette smoke, platforms were devoid of yellow lines or tactile paving. On the roads, cars had no seatbelts or airbags, steering columns were solid bars aimed at the driver's chest, bodies were thin steel without impact protection. Road death numbers were much higher than now, despite many fewer vehicles on the roads then.
In part I think that in the aftermath of WW2, many people had seen death and injury all around them on a scale unprecedented in a civilian population, and were simply inured to it. Industrial accidents in particular were just accepted as an inevitable part of life until H&S legislation in the 1970s and since. But I'm getting a bit away from dome car windscreens now.
