If you are talking about the final infantry attack of the day, the simple reason is that they met a line of Allied infantry and artillery that poured withering volleys into their ranks (they were advancing in large, unwieldy squares) and sent them packing.

For most of the battles in which they, the Old and Middle Guard, had participated throughout the Napoleonic wars, they had been held back in reserve - a fantastic morale boost for the rest of the French army, knowing that these old veterans were always to the rear, waiting just in case. Whenever, the Guard had actually been thrown into action, the opposing forces usually crumbled away. The Guard had never faced the British army, and a combination of this Thin Red Line, and devastating Dutch/Belgian artillery fire, must have come as a true shock. A number of Allied cavalry units had been largely untouched in the days fighting, and as the Guard fell back, these horsemen harried and pushed, ensuring a general disintegration of the French army.