I suspect a lot of it is pure population density. The US needs to make downtown areas pedestrian and bike friendly, and then allow high density development near the centre. That makes the bike lanes and the bus network self supporting.

In London there was a very big push to extend the bus network about 15 years ago, by the left-wing mayor (who had been given newly created powers). He also introduced a congestion charge for other vehicles. Both policies were quickly supported across the political spectrum. In central London few people use cars now, it is just far more of a hassle. The last mayor expanded bike lanes a lot, and now bike journeys are on course to overtake car journeys in central London.

The big development definitely helped to establish the network, but it happened on top of high density (in the number of people working in the centre, if not actually living there). And now we are getting much more housing closer to the centre.