Communications weren't quick
It would have taken hours for the news to reach all the units on the western front. Radios were not in widespread use, so many telephone calls would have had to be made to many headquarters. They would then have had to send messengers to all their sub-units that didn't have telephones.
If one side stops shooting, and the other doesn't, fighting will just restart
There would be no way to guarantee that the word of the cease-fire, spreading separately on the two sides of the lines, would reach the same areas at the same time. So if side A got an order "Stop fighting immediately," and side B hadn't heard, A would stop, and B would carry on, causing casualties on side A. In fact B would be more effective than usual, because they weren't having to look out for enemy fire. Pretty soon, A would start firing again, because being shot at while having the means to shoot back, but being forbidden to do so is very hard on the nerves.
So it was necessary to give some notice if everyone was to stop simultaneously.