The Federation colonists to that region are pretty recent arrivals, it's clear they've been there for one generation at most, if that, so it's not like the region was their "ethnic homeland" they had been living in for millennia. Also it's not like they fled an unstable, impoverished, over populated nation for new opportunity, the Federation is a massive post scarcity society. There's probably hundreds of planets in Federation space the colonists of the Demilitarized Zone could have been repopulated to without literally no discomfort, there's probably regions vacant enough you could move the ENTIRE population in one bloc so you wouldn't even be disrupting their community.

Essentially these people wanted to keep a brutal war going because they got "sentimental" about some planets they showed up to a few years ago.