Why was the Estates-General reconvened after a century and a half?
Louis XIV had ruled as an absolute Monarch without interference from the Estates General, but his reign was expensive, not only because of the lavishness of his estate at Versailles, but also because of the many wars in which he engaged. Under the French system of taxation, only the Third Estate (the peasantry) paid taxes. Rather than call the Estates General into session to raise taxes, Louis sold titles of Nobility which became hereditary. These were the "Nobility of the Robe," as opposed to the old traditional "Nobility of the Sword." The end result was to lower the tax base and put even greater pressure on the peasants. Louis saw the end coming, and is said to have remarked, Apres Moi, le Deluge (After me, the flood.) By the time Louis XVI ascended the throne, France was so far in debt from its many wars, including its participation in the American Revolution, that its income was insufficient to pay the interest on the debt. Louis XIV had no choice but to raise taxes, and he could only do so with the consent of the Estates General. It was not a situation of his own creation, but one that had been brewing and getting worse because of the policies of his predecessors.
In 1789, King Louis XVI called the Estates General to meet for the first time since 1614. The reason he did this was to try to get more money for his government. His government was in bad economic shape even though the country was rich. It was in such bad shape because most of the wealth of the country was held by the nobility and could not be taxed as much as the king would have liked.
Because of this problem, King Louis needed to call the Estates General. the point of calling them was to get them to agree to more taxes on the wealth of the nobility. Sadly (at least for the monarchy), calling the Estates General ended up provoking the events that caused the French Revolution.