Because there were bigger, stronger, older nations that would have loved to get their hands on that rich territory that the United States covered ... France, England, Spain, etc. Washington knew that the young USA could only survive as a nation if the people remained united, and all those things ... taking sides in international conflict (not remaining neutral), national debt, sectionalism (north against south, east against west, old states against new states), political parties (Whigs against Tories, Republicans against Democrats), are things that could set groups of the people against each other.
