Because it would cause too many paradoxes. Although time can be rewritten, the series has established there are "fixed points" that cannot be changed because they impact too many events that happen after, and the Doctor's past contains many. Creating a serious paradox can destroy time and/or attract the Reapers seen in "Father's Day". For more information about time paradoxes, see this article on Wikipedia.
An apparent exception to this seems to be the occasions where Doctors have met each other. However it has been strongly implied in at least two episodes that the Doctor either loses his memory of the meeting, or has it repressed somehow. The two episodes are "School Reunion" in which neither the Tenth Doctor nor Sarah Jane Smith remember meeting during the events of "The Five Doctors" (as far as they're concerned they haven't met since "The Hand of Fear", though Sarah Jane still has the K9 she received in "A Girl's Best Friend" which also appeared in "The Five Doctors"); and "Time Crash" in which the Tenth Doctor does not remember how his fifth self learned how to solve their dilemma until the Cloister Bell suddenly rings. The Eleventh Doctor has also gotten away with meeting himself (as in the Eleventh Doctor) on multiple occasions, even carrying on a conversation about River Song during "Last Night". In "The Day of the Doctor", the War, 10th, and 11th Doctors meet (and all incarnations of the Doctor are present as well), and it is established that only the latest incarnation retains memories of the meeting.