Answer:
Because after the loss in Russia, he needed to retreat, and new coalition of European countries was created. After the battle of Leipzig in reign was practically finished.
Step-by-step explanation:
After the Leipzig he retreated to France, and great forces decided to remove him from power. The was sent as a prisoner to island Elba. He spent some time there, before he escaped and recaptured the throne. He reigned again for 100 days, before finally being defeated in the battle of Waterloo.