But you could say the same of myriad situations throughout Who.
Why didn't he save Adric? Not like the mass of one boy was going to be the deciding factor between the dinosaurs being wiped out at surviving? Hell he could rescue Adric then drop him off again as an old man to be blown up.
Why didn't he rescue Peri?
Reinette? Well he could just pop back and see her, as for the letter, heck I can envisage an amusing scene where the 11th doctor stands behind her dictating bits of it.
I think the Doctor just knows sometimes, knows when things are at an end, when he can't interfere, and as someone else said, both the Ponds and the Doctor knew time was up, they just needed an excuse.