A satellite orbit has an inclination. If you launch it lined up with the equator then it will always stay over the equator. So you choose how far the satellite will wander north and south by adjusting the inclination at launch. The less the inclination the fewer satellites you need for coverage. So it is a tradeoff. Obviously the decision was made to only cover up to the place that population density falls off in the northern direction.

Added: Because the satellites spend more time at the northern and southern extremes they get denser coverage there. That is likely why they are initially offering coverage near the US/Canada border.