King Louis XVI failed to handle and control the politics of France like the past monarchs did. He didn’t pay attention the quarrels of prominent political and financial individuals in government. He sent funds to support the American Revolution which eventually put France in a huge debt.
The revocation of the Edict of Nates was another costly mistake he made, the protestant had no choice other than to convert or leave the country. His failed to prevent the mass departure of Protestants from France, the protestant had prominent roles in the economy, when they left France’s economy suffered greatly.
He allied himself with the aristocrats that had fled France and let people that the French people disliked support him. Louis XIV and his family fled during the storming of Bastille, this made the people know that he is not trustworthy. He wanted to retain the old regime so he could have power and the support of the nobles. His actions lead to the beginning of the French revolution.
Short answer - greed and selfish disregard of the basic needs of the populus. Probably ignorance of how people lived caused this. The over-protected, indulged courtiers and royals had not been brought up to consider others.
The enormous difference between the riches of the palaces and the gentry and their callous disregard of suffering from poverty, poverty caused by royal demand for luxuries reached boiling point and once their blood was up, the people were not going to treat their past lords lightly. Disregard others' well-being at your peril!