Why does Ralph say that he is in charge on the island?

Ralph tells the naval officer that he is in charge because he was the elected leader of the boys. Jack simply declared himself leader using a combination of bribery and force but Ralph remained the only truly elected leader. To have allowed Jack to claim leadership would have meant that Piggy and Simon had died in vain. Ralph gave their deaths some meaning by claiming leadership and by doing so claiming victory on the behalf of right, adherence to laws and justice over the savage anarchy and violence which Jack represented.