They initially choose Ralph for "his size and attractive appearance". Jack wasn't chosen because he was "ugly". It's normal for people to want their leaders to be attractive and charismatic, but that's not always practical. Ralph was chosen because he looks what the kids think a leader should look like. Later, the boys choose Jack out of fear and a desire to be freed from responsibilities. Once again they choose their "boss" with the simplest means and not with the best one. The reasons that lead the boys to choose him have allowed Jack to become an "irresponsible authority" and make him somehow synonymous with the dictators that people have voluntarily chosen around the world.