They did this to turn their attention away from the death plague ravaging the country.
Explanation:
The country under the reign of Prince Prospero had just been undergoing a tough time with the ravaging effects of the plague which was taking people's lives. Instead of the Prince to find a way to handle the menace in the society, he locked himself and his friends away in the safety of the palace so that they are not affected by the plaque.
He then throws a party to channel their attention off the problem. That action had dire consequences as it provided the opportunity for Red Death to gain access to the palace and do away with their lives.
