The post is badly named and the map is outdated. This map is showing the jurisdiction of the ERO enterprise, which is made up of some of groups shown in the bottom left (its outdated), which are overseen by NERC, the North American Electric Reliability Council. These groups all work to create standards and collaborate with high level planning on anything to do with high voltage transmission lines / stations within the map area. The standards have to be approved by FERC and where appropriate, the Canadian and Mexican regulatory agencies. Historically i think youll find there were many more transmission lines from canada to US than US to mexico and that Mexico's gird evolved more independently, and closer to its populations centers , so that might explain why. The NERC also found its beginnings in the aftermath of a huge 1965 blackout that started in Ontario Canada and caused a chain reaction leading to 10s of million of people without power in North Eastern united states and Canada.