I’m guessing it has something to do with the angle at which they’re drilling. They can’t drill something at an angle too acute, or they’ll risk a collapse of the tunnel. Whereas if they drill straight down and gradually angle it toward the old well, the tunnel can withstand greater pressures from the sea floor.
Mind you, I’m not a professional, just an observation.
You don't need to be a professional to appreciate the enormity of the task.
Deep water drillers ...."they da man".