Well he can, he just can't change anything. This is because unlike most points in time which are in flux, where a new timeline can be created to replace the old one, fixed points can't be replaced. As such, if one does attempt to change a fixed point, both versions of the events simply happen at the same point in spacetime, causing time to stop all together like a broken record. At this point the entire rest of time simply begins pile on top of itself at this one broken point. This is explained by the Doctor to Amy when River tries to rewrite a fixed point in "The Wedding of River Song", and this very thing happens.
For instance, in "The Angels Take Manhattan", when the weeping angel sent Rory and Amy back through time, their whole lives were immediately fixed into place. As such, the Doctor could never visit them, as it would rewrite their now fixed lives. Since they died of old age at some point prior to 2012, they're now effectively dead from the Doctor's perspective as a time traveler.