This is a very broad question and many answers can be given. In my opinion, there are two very important factors.
First, for there to be independence, a nation or people need to fight for it. In other words, there is a certain amount of military prowess that is needed. In light of this, we can say that the will of the people to fight England for its independence was an important factor in America's independence. Also we can say that this military development also helped tremendously in the future of America as well. After all, we are still the strongest military power in the world.
Second, the geography of America was also an important factor. The fact that it was far away from Europe helped in establishing independence. The distance made it hard for any type of foreign invasion to be successful. Moreover, the land was rich with resources for those who settled to use.