Because Rose needed a doctor to grow up with.
The question still stands: Why did he take them to Bad Wolf Bay?
Probably there was something that made Bad Wolf Bay a comparatively easy point of access for inter-universal communications. That was where he could get a message to Rose in "Doomsday". Since he was taking them to the parallel universe, that was the available point of entry.
Note, though, that "comparatively easy" doesn't mean really easy. He needed the power of a supernova just to make a fairly brief phone call.