I think because he expected her to have lived through and remember the incident at Canary Warf where the Cybermen and Dalek's were pulled into the void and Rose was lost to the other universe. read more
Time travellers can still remember events that were unwritten, as long as they are not part of their own history, but no one else can. This is why Amy remembers the missing Clerics, while the remaining Clerics do not. Since Amy wasn't a time traveller when the Dalek invasion was unwritten, she doesn't remember the Daleks. read more