Factual error: Harry says to Sirius in one of his letters that Dudley chucked his Playstation out the window. However, Book 4 takes place in 1994, and the PlayStation wasn't launched in Britain until 1995.
Factual error: In Hogsmeade, Sirius implies that at a certain point, Snape's attendance at Hogwarts overlapped with Bellatrix Lestrange's. But given their dates of birth, Bellatrix would have left school two years before Snape started.
Answer: Because Crouch was a notorious dark wizard who was supposed to be locked up in Azkaban. Crouch had to be free to take over the life of Professor Moody. Even if they could find another person, Voldemort still wanted Harry for the ritual.
Greg Dwyer