Why would the Spanish government let the Catalans declare independence regardless of the outcome of the illegal referendum?
Read more about the right of self-determination here: http://www.unpo.org/article/4957
Edit: they've neve been independent, that's more of the bs that we're hearing these days.
I think you may be confusing the pattern of colonization in British North America with colonization more generally; the pattern elsewhere was more often of subordination to an imported elite rather than displacement.
Perhaps, if that is what the people in the South (and not just those in positions of relative legal privilege) wanted.
But I doubt that, for example, the 55% of Mississippi's population that was in slavery was particularly energized about secession to preserve the right of their masters to keep them in slavery, though.