Though Ralph desperately wants to be a leader, he isn't able to feel the pulse of the boys on the island the way Jack is. Jack takes advantage of the fact that the boys are scared, that they want something to be fighting against, and that they are hungry and want immediate fulfillment. So Ralph misses this and his pleas for order and rules and that sort of thing are gradually less and less relevant to a group of boys that are terrified about a beast and worried about their ability to get enough to eat, etc.
In short, Jack has charisma and leadership ability, even if he doesn't make logical decisions like Ralph. Because Ralph lacks these, as the bonds of civilization weaken, so too does his hold on the boys.
In my opinion, Ralph loses power over time because of the fact that, according to Golding, people have an evil side that overcomes their tendency towards civilization. This is the main point that he is trying to make in the book.
The boys begin to follow Jack, in my opinion, because his way sounds more exciting. The idea of hunting and being wild is much more interesting than stolidly trying to make huts and tend signal fires. I think that the boys also start to follow Jack as Ralph's ways seem not to be working. The boys are not getting rescued and the huts are falling apart, for example. So Ralph's ways are less interesting and they are not getting results either.
The longer the boys are on the island, separated from the rules of society, the more savage they become. Ralph represents order, the adult world, rules, which at first, when the boys have not been separated from society for so long, appeal to them. They take comfort in setting up a mini-democracy on the island, with the calm, intelligent Ralph as leader.
However, the longer they are left to the evil within them, the less and less Ralph is able to control them, and the more and more they become attracted to the violent Jack and his hunter mentality. The more they are attracted to Jack, the crueler they become as well, until finally, they turn into savages and turn on each other, killing Simon and then Piggy.