Final answer:
The main reason for the calling of the Estates General was economic instability, as France was facing severe financial difficulty, which the monarchy sought to address through new taxes and reforms proposed with the convening of the Estates General.
Step-by-step explanation:
The calling of the Estates General in 1789 was chiefly due to economic instability. King Louis XVI summoned the Estates General as a response to the Crown's financial crises, brought on by years of poor harvests, support for the American Revolution, and unsustainable debts. The outdated structure of the Estates General, where each estate had one vote, led the Third Estate to demand more representation, influenced by Enlightenment ideas of rights and equality. This demand was ignored, prompting the Third Estate to declare themselves the National Assembly, sparking the revolutionary movement that led to the French Revolution.