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As explained (albeit very briefly) in "The Day of the Doctor", such memories would have caused a temporal paradox & the timestream protects itself from such paradoxes by ensuring that events that shouldn't be remembered aren't remembered. The situation in which the "War Doctor", the Tenth Doctor & the Eleventh Doctor (&, eventually, 10 other incarnations) teamed up to save Gallifrey only came about because the Doctor had not remembered it happening.

This "memory editing" effect has been shown several times before, when different incarnations of the Doctor have worked together. In "School Reunion", for example, it was clear that neither the Doctor nor Sarah Jane Smith remembered their meeting on Gallifrey in "The Five Doctors". In terms of storytelling, of course, this is necessary otherwise dramatic jeopardy is destroyed if the Tenth Doctor, for example, knows that he will regenerate into the Eleventh someday. "The Five Doctors" and "Time Crash" do hint that these memories return if another multi-Doctor scenario occurs (the Third Doctor, for example, is able to describe the Fourth Doctor's appearance to Sarah Jane in "The Five Doctors", and the Tenth Doctor - albeit after his memories are unlocked by the ringing of the cloister bell - remembers how to save the day in "Time Crash")