You seem to be quite mistaken, even with your edit... Central America, the geographic region, is part of North America, the continent. Not counting the island nations of the Caribbean, there are still 10 countries - Canada, USA, Mexico, Belize, El Salvador, Honduras, Guatemala, Nicaragua, Costa Rica, and Panama. So that is still below average, but there are a lot more than two.

So I think you might want to rephrase your question. It isn't why are there two countries in North America, but why did the USA and Canada (and perhaps Mexico as well) manage to get so big and not end up as a number of smaller states.