I agree with you there. The Maquis should have left the colonies. It's the 24th century in Trek, re-settling elsewhere in the Federation should be easy.
I think that the colonists were so stubborn for two reasons.
First, they do attract adventurous, not particularly risk-averse people. It's kinda like with the settlement of the New World, but the selection effects are stronger as there are no economic incentives in the world of Trek (America was land-abundant while labour was scarce so wage to land rent ratios were higher in America than in Europe). So in the core of the Federation people we have these (I am exaggerating) phlegmatic, risk-averse hobbits who caring about a peace
ful and quite life. Naturally the adventure types on the border are more trigger-happy and less concerned about their own life.
Second, we all have read our Ernest Becker. People need an "immortality project", people need to pursue something which (they can pretend) continues to exist after they are death. Gilgamesh and the wall around Uruk, a writer living on via his books, having children. For people in the "outer rim" their life's work is in the places they made habitable so they don't wanna give them up. As you said, life elsewhere should be easy but unlike animals we humans are not simple hedonists. Once some of them fight in the Maquis their new immortality project might have become giving up their comforts or ultimately their life to fight for what they perceive as a good cause (MLK's last speech neatly shows how somebody thinks and feels in such a situation).