Why does Charles Wallace get left behind?

Charles Wallace is a main character in Madeleine L'Engle's A Wrinkle in Time. He, his sister Meg, and their friend Calvin travel to Camazotz with Mrs. Whatsit, Mrs. Who, and Mrs. Which. The children are left to travel to Central Intelligence without their guides. They do not make it very far before a teacher finds their constant questions inappropriate and sends them to be reprocessed.
During reprocessing, the children encounter a strange man who speaks oddly and makes them uncomfortable. He appears to be a mouthpiece, and the children speculate he may be hypnotized, like the other adults they have come across in Camazotz. The man hypnotizes Charles Wallace, and he leads the children to Mr. Murry. Meg and Calvin rescue Mr. Murry, but Mr. Murry is not able to help Charles Wallace escape the hypnosis.
Charles Wallace leads the group to IT, a giant, pulsating human brain that is controlling all of the hypnotized people in Camazotz. IT almost begins to control Meg and Calvin, but Calvin encourages Mr. Murry to "tesser." They escape Camazotz. They are forced to leave Charles Wallace behind, though, because he is so taken by IT that they cannot connect with him enough to take him along on the "tesser."