Why does Meg have to be the one to rescue charles wallace, rather than Cal or Mr. Murry? I really didn't understand. please explain :)

Because she was the only one who had been with Charles Wallace long enough to properly love him as her brother. The thing that released Charles Wallace from IT was love.

She is the one who is closest with Charles and the one who is therefore most likely to get through to him, to bring him back successfully.

Meg has to be the one to rescue Charles Wallace because her bond with him is the strongest. Charles had only just met Calvin and Mr. Murry had been gone since Charles was a baby. But Meg had been with Charles for his entire life. Meg and Charles had a very, very deep connection; to the point where Charles could almost read Meg’s mind. They loved each other a lot and that means that Meg is the one most likely to get through to him and rescue him from IT.

Honestly idk