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It takes too much energy (the TARDIS pretty much had to burn up a sun so the Doctor could say goodbye) and is too dangerous to both universes; as mentioned in "Army of Ghosts"/"Doomsday", Pete's World is pretty much burning up because of the link with "our" Earth.

Plus, according to the Doctor, the walls of reality have since sealed, which prevents him from going to any alternate universe. ("Journey's End")