Waterloo

In 1814 French Emperor Napoleon Bonaparte, facing certain defeat at the hands of Britain, Austria, Prussia and Russia (the Sixth coalition), abdicates at the demand of his marshals. He is banished to Elba with 1,000 men, but escapes and returns to France. Ney, now serving the monarchy of Louis XVIII of France, is tasked with recapturing him, but he and his army defect to Napoleon. King Louis flees, Napoleon triumphantly enters Paris, and the European powers declare war.
The Prussian von Muffling interrupts the Duchess of Richmond's ball to warn the Duke of Wellington that Napoleon has invaded Belgium to defeat the Allied forces before they can unite. Realising that Napoleon has got between himself and the Prussians, Wellington decides to halt the French at Waterloo.
The French fight the British to a draw at Quatre-Bras, but defeat the Prussians at Ligny. Field Marshal BlÃ¼cher rejects the advice of his Chief of Staff, General Gneisenau to retreat and instead moves north to Wavre to keep contact with Wellington. Napoleon, enraged that Ney has let Wellington withdraw to ground of his choosing, directs 30,000 men under Marshal Grouchy to pursue BlÃ¼cher and keep the Prussians from rejoining the British, while he leads his remaining force against Wellington.
The battle of Waterloo, delayed to let the ground dry after the previous night's storm, starts shortly after 11:30 am with cannon fire from the French. Napoleon launches a diversionary infantry attack on Wellington's right flank, the Chateau of Hougoumont, but Wellington refuses to divert forces. Napoleon then attacks the allied left with d'Erlon's infantry corps. General Picton successfully halts the attack but is killed. Ponsonby's cavalry brigade, the renowned Royal Scots Greys, pursue the French, but go too far across the battlefield and become isolated from the rest of the Allied force, and are thus cut to pieces by Napoleon's lancers. Ponsonby himself is killed.
Napoleon realises that troops spotted emerging from the woods to the east are Prussians (BlÃ¼cher's army), not French (Grouchy's force), but keeps this from his army. He then suffers stomach pain and withdraws temporarily, leaving Marshal Ney in command. Ney misinterprets a reorganisation of the Allied line as a retreat and leads a cavalry charge, which is repelled with heavy losses by allied infantry squares.
Napoleon returns and rebukes his marshals for letting Ney attack without infantry support. However he hopes that Wellington's line has been worn down. The British farmhouse of La Haye Sainte falls, and Napoleon sends the Imperial Guard for the decisive blow. As they advance they are repulsed by Maitland's Guards Division, who were lying unseen in the grass on the reverse of the slope. The repulse of the Guard devastates French morale, and the arrival of the Prussians makes matters certain. After refusing to surrender, the Imperial Guard squares are annihilated with close range artillery.
After the battle, Wellington wanders among the piles of dead, lamenting the cost of victory. At the same time Napoleon, who had declared that he would die with his men, is dragged by his marshals from the field and later departs in a carriage for Paris.

Question: Why the battle of Waterloo delayed?
Answer: let the ground dry