I think this is part of it, but there are also issues of infrastructure as well. Many aging eastern cities, with their challenging (read: hilly) topography, make it prohibitively expensive to radically redesign roads.[0] Additionally, I think it would be "federalize" such efforts, as it would be easier gradually redesign roads in a city, or even a part of a city, rather than attempt to broadly dictate policy for the entire country.
[0] That said, it is possible to make the streets safer, even when geography and economics are obstacles. My own town has been doing this, though often it is piecemeal.
Great point. It's made more complex by artifacts of state law and home rule for political subdivisions.
For example, in New York, state and US highways (ie. State Route 5, US Route 20) are maintained by the State DOT -- unless you're in a "city". Areas that are organized as towns (think Hempstead, NY on Long Island with 300k+ people) get state services, areas organized as cities (Albany, NY with 90k people) typically do a lousy job at it.
The Federal government has been able to update things like street signals, etc. But fundamental things like road conceptual design, breaking out bike lanes, etc are going to be very difficult issues to handle without local control.
There are more people in NYC than Sweden, so the size and scope of the problem in the US is several orders of magnitude more complex.
Hilly topography doesn't really make it prohibitively expensive to make 2+1 roads, or at least they're relatively common in Finland. (Although 2+1 roads are rarely separated by fence here).
Even hilly roads tend to have straight sections that can serve as the 2+1 area.
[0] That said, it is possible to make the streets safer, even when geography and economics are obstacles. My own town has been doing this, though often it is piecemeal.