Prospero chose courtiers instead of ordinary citizens to go into seclusion with him because he was selfish and arrogant.
When the Red Death attacked his kingdom, Prospero did not do anything. He waited until half of his people were dead, and then went into seclusion with one thousand of his closest friends.
When his dominions were half depopulated, he summoned to his presence a thousand hale and light-hearted friends from among the knights and dames of his court, and with these retired to the deep seclusion of one of his castellated abbeys.
Prospero was “sagacious” but also selfish. He wanted to have a big party. He invited his courtiers, or the wealthy and important people of the kingdom, because he deemed them worthy. He really did not care about the rest of the people in his kingdom. He did nothing to prevent them from dying.
The abbey was amply provisioned. With such precautions the courtiers might bid defiance to contagion. The external world could take care of itself. In the meantime it was folly to grieve, or to think. The prince had provided all the appliances of pleasure.
The Red Death killed incredibly quickly. It was a terrible death, but fortunately a quick one. Still, people were contagious and the disease spread fast. The people locked up in the castle could not escape it. Eventually, death comes even to the rich. Prospero and his friends found that out the hard way.
Prospero cared nothing for the ordinary people in his kingdom. If he was a good ruler, he would have done everything he could to stop the spread of the disease. He also should have helped the suffering. Instead, he put himself above them, and did not care a bit about their deaths. He paid for his arrogance and so did those who followed him in his folly. The abbey was nothing but a delusion and an illusion of safety.