The superficial, first answer to the question is that Jack promised the boys they would have meat to eat. After the time on the island to this point, eating only the fruits they have found growing there, the appeal of something different - and especially the idea of having meat - was a tremendous attraction.
On a deeper level, the transfer of loyalty from Ralph to Jack followed the transition from civilization to savagery in the behavior and outlook of the individuals and the group as a whole. In earlier parts of the story, the boys were still struggling to maintain relationships and manners and attitudes based on what they had been taught in their schools and among their family members. Now, with the passage of time, the animal instincts buried within the boys were coming out. The urge to hunt and kill in order to survive is becoming stronger, overpowering the refinements of behaviors learned under different circumstances.