Because there is no North American Union. If Quebec seperates from Canada, the subsusequent renegotiation of NAFTA from a base of 4 is less likely to happen. Candians don't have the legal right to work freely in the US without permit, Basque do in Spain because of the EU.
It's the exogenous influence of the EU breaking down the economic incentives of union that makes fracture possible. That's my point. The power of the EU and the minority regions grow in tandem, because they undermine the incentives of union with the national state. And I'll give my consent to any government that does not deny a man a living wage-Billy Bragg