Because he fears that they will not heed his call. He knows that if he calls and they do not answer then all pretence of any authority he might hold over them will have gone for good.
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He takes his group to the platform because Piggy wants to get his glasses back from Jack and the other hunters. And Ralph wants to get his authority back.
Jack attacked Piggy rather than Ralph because he knew Piggy wouldn't fight back. If he had attacked Ralph there was a good chance that Ralph would have beaten him and Jack couldn't take the risk of losing face in front of his own hunters.
When the conflict between Ralph and Jack gets bigger, the boys devide in two groups: Jack's and Ralph's group. the first one is more savage than the other. Ralph's group is only made up of Ralph, Simon, Piggy, Samneric and some of the little ones.
Ralph wants to get Piggy's specs back and Piggy wanted to kill Jack there and then.
all but jack have gone swimming