He couldn't lower his guard, needed to use everything he had available if he wanted to win without risking a premature defeat in this tournament that would damage not only his pride a bit but also give the nobles reasons to talk.
While they could no longer deny he deserved the rewards, it could generate more difficulties in managing his territory... And especially, he wanted to indulge his parents by winning, didn't want to betray the new trust they'd placed in him.
But there was also a more analyzed reason for this decision...
He wouldn't run out of secrets or options.
He'd keep thunder, ice, and other more advanced combined elements as hidden cards for the future.
Light and darkness would have to be shown in all his beasts eventually anyway because they were the fundamental base to many of the techniques he planned to develop later. He could reveal them now and perhaps improve them soon so he wouldn't be left so exposed.
