In addition, the Netherlands is very flat, and I've found this makes a huge difference. The steepest "hills" I've encountered while biking around cities there

In addition, the Netherlands is very flat, and I've found this makes a huge difference. The steepest "hills" I've encountered while biking around cities there are the bridges over canals. Cycling is extremely efficient on flat roads and doesn't take much energy if you're not trying to go too fast. But as soon as you get to a slight incline you notice the difference (especially in the summer when you're concerned about sweating). I consider myself fairly/very athletic and I definitely noticed the difference (how easy it was) when I biked around there. Obviously, these aren't the only reasons cycling isn't as popular in the US. There are certainly US cities which are just as flat and mild.