Explain why Meg, rather than Cal or Mr. Murry, had to be the one who rescued Charles Wallace.

Meg certainly seems like an unlikely choice as the one to return to Camazotz to rescue Charles Wallace. However, as we learn in chapter 12, Mr. Murry and Calvin can't do it. First of all, Mr. Murry, although he is the most experienced with tessering, is told flat-out by Mrs. Whatsit that he cannot be allowed to go. They all know that his attempt would end in failure. Calvin is similarly told he cannot go when he volunteers. It is explained to them that, should they try to rescue Charles Wallace, they would likely be pulled into IT as well.
It is Meg who, rather reluctantly, insists that it must be her who will go to save her brother. As Meg explains,

It can't be anyone else. I don't understand Charles, but he understands me. I'm the one who's closest to him. Father's been away too long, since Charles Wallace was a baby. They don't know each other. And Calvin's only known Charles for such a little time. If it had been longer then he would have been the one, but—oh, I see, I see, I understand, it has to be me. There isn't anyone else. (page 188)

It is this close connection between the siblings that allows Meg to save Charles Wallace. Her love is able to reach through the grasp that IT has on the boy and pull her brother back. In the end, it is her love for her little brother that IT is powerless to fight against.