Bringing the population to 1 million

[Phalanx]

I don't think it's 'best technology' per se, I think it has more to do with industry and economy. North America was shaped largely by the prevalence and power of the automotive and energy (oil) industries from the 30s-70s.

Manufacturing jobs were plentiful, fuel and cars were cheap, and both industries had a lot of sway. The distances and relatively low population density between cities made the infrastructure required for rail prohibitively expensive at the time.

If you look elsewhere (Europe and Asia) things developed differently.