The USA started with a much larger loading gauge, making double-stack easier to achieve in recent decades, especially in the open Midwest, where there are few major tunnels or even many overbridges. Older railroads in the east often didn't have such generous clearances, which even now limits the range of Amtrak's bi-level superliner cars, as well as double-stack freight service. Eastern railroads have since undertaken significant clearance works on selected routes for such freight trains, but superliners are still excluded from Amtrak's own Northeast Corridor (NEC). There are so many significant overbridges and tunnels on this route through some of the densest populations in the USA that the necessary works would be impracticable.
The cities of the eastern seaboard are useful in considering the difficulties such gauge enhancement would face in the UK. Most of the NEC is already grade-separated with few level crossings. That's similar to UK mainlines, which pass through many long-established large settlements. Rebuilding countless urban overbridges for a significant clearance enhancement would be extremely expensive disruptive and unpopular.
Another thought. Even if double-stack height were achievable on existing tracks by rebuilding structures, the side-to-side sway component of the 'dynamic envelope' of such tall vehicles would increase, requiring a greater minimum spacing between tracks, without the vehicles themselves being physically wider.
