If localities have self-determination, it results in a country full of mini-laboratories where we can figure out what works and what doesn't.
Granted, in the localities that practice oppression and hatred, things will be miserable, but at least people can leave. If enough people leave, the people who stay behind will become marginalized and poor and, in general, suffer. Everyone else can then see the terrible results from that laboratory and do something else.
If we don't give localities self-determination, instead forcing one flavor on the entire nation, we don't get all these laboratories. If we adopt a model that leads to suffering and deprivation, the entire nation is screwed.
The entire nation, in other words, can become Portland.