I'm not sure if your're asking why Meg had to go to Camazotz the second time, or what she had that IT didn't have. Both of these things make Meg "odd," or rather, set her apart from Calvin and her father, Mr. Murray.
The reason Meg was the only one who could go back to Camazotz to save Charles was because she understood Charles better than anyone else. Meg is closest to Charles and they have the most intimate relationship. Meg describes this on page 188 when she realizes why she must be the one to go.
On page 195, Mrs.Which says to Meg, "You have somthing that IT has not." Meg, at first, does not know what Mrs. Which means. However, on page 200, Meg realizes that what she has is love for Charles. It does not love Charles, but she does. It was Meg's love for Charles that finally broke his bond with IT.
Hopefully this answered your question. Good luck!