Given that Golding's purpose in many ways was to respond to the book "Coral Island" by Ballantyne and the depiction of British boys as being more civilized than the people they run into, I have to assume that Jack succeeded in order to demonstrate the tendency of men or boys to give in to the evil inside of them.
The second reason is simply the fact that Ralph lacks a great deal of the instinct to be a true leader. Though he wants very badly to be respected and to end up the leader, he misses crucial things and does not take advantage of some of the opportunities that Jack finds to win boys to his side.
I'm not sure that I completely agree with your statement. Ralph may have failed to secure his position as leader of the boys on the island, but he maintained his own civilized nature until the end when he was forced to defend himself from Jack and the others. Jack succeeded in much the same way Hitler did: He overthrew the powers that be and resorted to inhuman ruthlessness to maintain his position. In the end, when the civilized world returned to rescue the boys, Jack's rule came to a quick end.
I would say that there are two reasons for this.
First of all, Jack is more ruthless than Ralph. He is a savage and so he has no compunctions about using violence to get what he wants, for example. And he is willing to force people to do things; Ralph does not like to be a dictator in this way.
Second, I think that Golding is saying that the savage instinct is strong in people and can only be overcome by an established civilization. On the island, the kids have no established civilization. This makes it much easier for Jack to succeed than it would be for Ralph.