Why was it so difficult for Ralph and Jack to communicate with each other?

I assume that you are referring to the conversation between Ralph and Jack which took place in chapter 3: Huts on the Beach. Both boys are similar in some respects, they are both passionate and focused on what they believe to be important. Unfortunately they both find different things to be important and are unable to communicate this to each other. Ralph believes that getting rescued is of the utmost importance,. he sees the need to building shelters and have rules as ways to preserve order and civilisation. Jack is simply obsessed with hunting pigs, he wants to kill pigs and everything else fades into insignificance, everything else except his desire to be leader. Jack sees Ralph adherence to rules and order as simply inconvenient obstacles which stifle his own desires and needs.