Answer:
One reason is that the French monarchy really liked to spend money. King Louis XVI was always trying to keep up with the expenses of his wife, Marie Antoinette (nicknamed "Madame Deficit"). The people of France were split up into three estates: 1) The clergy, 2) The nobility, and 3) The peasants and the middle working class. The Third estate worked the most, got paid less, and had to pay the most taxes. Two bad harvest years put France into an economic "slump". Add that to the Madame Deficit problem and you've got a nearly bankrupt country. In response to this, good ol' Louis decided that it was time to raise taxes, which were already outrageously high. The Third estate revolted and decided to draft their own constitution. When the representatives of the Third estate tried to show it to the other two estates, they were ignored. So, on a tennis court, the reps vowed that they would meet there on that tennis court until they had gotten their very own constitution. That event was called the Tennis Court Oaths, for obvious reasons.