This is a good question. There are two main reasons why the boys do not follow Jack immediately.
First, in the beginning of the book, Ralph is already the natural leader of the group. So, the boys just stick to him. Moreover, when there is the thought that there will be a quick rescue, it makes sense to follow Ralph who not only has leadership qualities, but also counsels things that make sense in that context,
Second, Jack only becomes more attractive as a leader when the book progresses, because there is no rescue in view. Moreover, the community of boys begins to deteriorate as they want meat to eat and want to join the hunt. When "lawlessness" and chaos settles in and this becomes the ethos of the community, Jack is a better suited leader. In other words, it is only when the context changes that Jack is seen as more of a leader than Ralph.