The order of all stop was given after the collision. After that the lifeboats were being lowered and the boilers were shut off.

It would have been unrealistic to relight the boilers and then start moving the ship after the Californian was spotted--and they didn't know what it was since the wireless operator was asleep and no one was there to monitor the transmissions from the Titanic.

Then what do you do with the lifeboats you've left behind in the water. If you don't make it the ten miles to the next ship, and you sink--you now have two groups of lifeboats that have no idea where the other one is.

Also, at this point, the ship was sitting so far down at the bow, the same thing would happen to the Titanic as happend to the Brittanic--it never reached the shoreline because its downward angle made it dig into the water.