They did not understand. The bread that Moses gave them was a type, it was a prophecy, it was in symbolic form a prophecy of how Christ would one day come.
Whenever Christ worked these miracles, he explained them in terms of salvation. Now he had not gone to Calvary yet. They had to trust him in expectation that he would redeem. ‘I am the bread of life’, I am the one who can provide you what nothing and no one else in this world can give you, reconciliation with God, forgiveness of sin, everlasting life. Trust in me as your Messiah. And when he went to Calvary, they understood this is how he has done it. Some of them may have understood before, some of them may have understood Isaiah 53, but very few. The disciples did not. As long as they trusted in him, that was good enough until Calvary. The great sign of salvation is there.