1. Inertia. It is difficult to switch measurement in people's heads. For example, I grew up in metric environment, but I did not drive car. So for me, local sizes are in meters, but car speed, distances are in miles :-). Home temperature is in Celsius, but grill or oven are in Farenheits. :-)

2. Cost. There is a direct cost (like remarking everything), and indirect -- all the chaos and errors because of mixing up units. To foot the bill is hard sell in Congress.

3. Peer pressure: because of size of economy, US can request whatever it wants from partners. UK had to move to metric when joined EU.

That said, I am very supportive of metric. We are paying price for the imperial units. Every car shop has two set of wrenches. (BTW, it is metal, production -- read climate impact. May be not big, but still impact we could avoid.) All the software must show controls for units. We need to train professionals (like doctors) to live in two worlds. And the list is endless. [Did I mention translation bugs? For example, outside temperature translation is 9/5 Celsius + 32, but difference between outside and inside is 9/5 Celsius. :-( -- real bug I caught in critical industrial control system. ]

I am not sure where does US stands today. For example, I think metrics is teached in school. Most food is dual marked. So, maybe, it is not rejection, but rather slow move?