I think an economic union is difficult to manage politically, because you need some sort of a central decision making body with representatives from each nation.
Can you imagine Canada and the United States being part of an economic union? We already have trade agreements with countries in the region and I think this is a great way to support our neighbors and strengthen the region economically, not to mention US businesses who can operate in these countries more easily.
But an economic union requires a country to loose some sovereignty and I'm not okay with that. It would be a nightmare to have to have other countries approve the economic policies in my country.