identity-no2 answered your question: So why couldn't the Doctor visit them again?
I think because of the ‘time disturbance’ mentioned at the beginning. First TARDIS landing was a risk already!
Ok but what disturbance? Does that mean he can't go back in time EVER for the previous 50 years? And if it's just New York then couldn't he go back to say, England, and take a boat to America or something? It just doesn't make sense to me (yet) so I can't appreciate the heartbreak.

















