It’s a question of self understanding I think. US states see themselves as states(in a sense of administrative regions) within a union. They do have strong regional identities, but no history as an independent nation or independent ethnic group (Excluding native Americans). So there is no strong drive to be a nation of their own. A strong sense of being “Americans “

Scotland and the UK on the other hand see themselves as states in a sense of nations that have “joined” in a union. Scotland was an independent country before. The ethnic lines have blurred over the centuries, but the Scottish see themselves as Scottish. To a much lesser degree they will refer to themselves as being from the UK.

So there is much greater empathy for a Scottish independence than for the potential Nation of North Carolina.