The US has mild and flat cities; none of them touch the Netherlands when it comes to cycling support.
I think the only missing piece in many cities is funding and prioritization.
I think the contributing factor is delivery people who use their bikes a lot, now another problem is that they are completely ignorant of any traffic rules, but can't have it all I guess
[0] https://www.wired.com/2015/06/copenhagenize-worlds-most-bike...
Of course, you need decent alternative infrastructure - buses, trains etc.
http://www.letsrun.com/forum/flat_read.php?thread=728621
(A discussion of how "hilly" Boston is with respect to the Boston Marathon.)
"If you live in Kansas Boston is hilly. If you live in Colorado Boston is flat."
What cities are those?