In the specific case of the US, it is the scale of the populated geography more than the number of people. This has created problems even in boring and non-controversial policy areas. For example, prior attempts to regulate highway speed limits nationally, which has since been devolved back to local control. At the scale of individual States, it works much more like it does in other countries.

Any national-scale policy will be poorly adapted for some major region of the US because it is simply too diverse geographically, culturally, historically, and economically.