By this point in the story, Jack is very much king of the castle. Having deposed what he regards as the "cowardly" Ralph, he's now firmly in charge, his dominant position ably symbolized by his occupation of the strategically important Castle Rock. The mountain, too, is very much his domain, and plays home to the mythical beast as well. By the same token, Ralph and Piggy are forced to beat an undignified retreat to the beach. The beach, like the Castle Rock and the mountain, is hugely symbolic of the changing balance of power on the island. It is flat and exposed, vulnerable to sudden attack by Jack and his vicious tribe of blood-thirsty choirboys. And if a rescue ship should suddenly appear on the horizon, it'll be much more difficult to spot, let alone contact, from the strategically weak beach than from the fastness of the mountain.
Ralph and Piggy tried valiantly to impose some measure of order upon their savage, unforgiving surroundings. They tried to make nature yield to the power of reason. But they failed, and their subsequent retreat to the beach symbolizes that failure. Jack, on the other hand, has gladly succumbed to the dark, cruel, irrational forces of nature, and has been rewarded with absolute power on the island. Literally and symbolically, Ralph and Piggy will attempt to keep the flame of civilization alive, but it won't be long before it's extinguished altogether.