First, there's just the willingness to invest in infrastructure, which is nearly non-existent in the US. Swiss people LOVE infrastructure, they are proud of their tunnels, railroads, bridges, dams, etc.
Take a look at the referendums over the past few years: universal basic income? No. More vacation days? No. Forbid speculation on food products? No. Define marriage as man/woman? No. Build a new tunnel across the Alps? Hell yeah!
But it isn't just about density. You can have mid to low average density, but still have a viable transit system.
Public transit doesn't just depend on the overall average density, but on the density of the area surrounding the transit stops.
Switzerland has reasonably dense cities and towns, with very-low-density areas in between. The SFBA has evenly-distributed low density (except for a few areas).
Increasing the distance between dense cores doesn't have as much negative impact on transit viability as spreading people out has.
The SFBA could easily solve this by changing the zoning laws around Bart, Caltrain, VTA and Muni stations.