What has Ralph fixed his mind on that the others seem to think of very little?

Ralph is determined that, once the boys have had their fun and settled down they should devote their energies to building a habitable community on the island a civilization with rules and regulations. Unfortunately, not too many of the other boys want to go along with him; they're too busy having fun. So Ralph's left virtually on his own as he manfully struggles to build shelters on the beach. Simon lends a hand, but it's a big job and Ralph's going to need a lot more help if he's to get it done.
We can understand why the boys, free from adult supervision for the first time in their lives, just want to enjoy themselves. But their unwillingness to help Ralph and Simon build shelters darkly foreshadows subsequent events in the story, when most of the other boys led by Jack will undermine and destroy Ralph's best efforts to found a civilization on the island.