At one point in the episode, there's the following exchange:
ROSE: (asks why they can't use the TARDIS instead of the time windows)
DOCTOR: We can't use the TARDIS. We're part of events now!
It's certainly a poor explanation, but it's one that's used occasionally throughout Doctor Who.
Whilst Doctor Who officially doesn't have canon, this (or a similar) explanation is often employed to justify why he can't simply travel back in time and undo lots of regrettable events. Apparently, this can result in paradoxes, the butterfly effect, and generally bad things. But hey, it's Doctor Who.
Out of universe, this is important because without it the show would be without consequence. Anyone who's hurt/dies/etc can be saved very easily. A variation of this explanation is the notion of time locks, which are placed on several important events including the Time War.
In-universe, this is known sometimes as the Blinovitch Limitation Effect
It is usually understood as having two aspects: firstly, that a time traveller cannot "redo" an act that he has previously committed, and secondly, that a dangerous energy discharge will result if two temporal versions of the same person come into contact. The first aspect is similar to a real-world physics conjecture, the Novikov self-consistency principle.
Some examples of this in the newer revival include in The Big Band (S5E13) where the Doctor brings two sonic screwdrivers into close contact and sparks are emitted which appeared to confirm they were the same sonic screwdriver at different points in time.