Because the original TV series established that he was still alive.

Wibbly-wobbly timey-wimey~

In the 20th anniversary episode, "The Five Doctors", it was shown that Rassilon had achieved a form of immortality.

he was alive because the time wars were technically still happening and the time lords used the Master as their final and last plan to escape the time wars, but I believe when the Doctor foiled the time lords plans the time wars were finally over so he can't come back anymore

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