1) Their army simply wasn't large enough to occupy enough square miles of territory in North America.
2) Distance. The American rebels had the "Home Field" advantage, while Britain had to maintain long supply lines back to the Mother Country.
3) The American Spirit. So long as the colonists were determined to resist, the British would have a difficult time retaining all the thirteen colonies. They had to break the American will to fight or at least disrupt America's unity to make it too painful for the colonists to wage a sustained rebellion.