It would look too artificial if none of the 7 Weasleys died.
Molly couldn't die as she had to kill Bellatrix.
Arthur couldn't die as Harry had already lost many father figures.
Ron couldn't die because of Hermione and Harry.
Ginny had to end up with Harry.
If Bill and Charlie died, it will not effect the readers.
Percy couldn't die as he had just reunited with his family.
This leaves the twins. And if both of them died, it will be less painful as they lived together and died together. To make it more painful the Author killed one of then, i.e., Fred.
Also, the twins were the fun makers. They joked on everything regardless of situation. The author wanted to make the Final Battle of Hogwarts sad. So, if Fred died, thre was no one to joke. (George was in grief.)
Poor George!!!