because for the spirit in Harry to die Harry himself has to be killed.Harry did not fully dieI'm not an expert, but I am pretty sure Harry doesn't become a horcrux until the 4th book when Voldemort uses harry's blood to come back to life.
I may be wrong, so don't quote me on it.Harry was not really killed at this point. Even if had been, it would not have been by Voldemort, but Voldemort's Horcrux. Had Harry died, he would have stayed so.because the oblect has to be destroyed, & Harry was not destroyed, He was healed when Fawkes cryed on him.
Rowling stated that the horcrux has to be destroyed BEYOND REPAIR