As told in vyasa bharatam as under :
After the sunset aswathama, kripacharya and one more warrior was walking away leaving duryodhana in the warfield. IN the twilight aswathama say the crows attacking the owl and once the sunset was complete the owls attacked back the crows. Aswathama felt that the power is moving concept and like the owl he can attack back the pandavas when they are asleep.
So he went to the pandavas camp, but he could not enter because there was a big Rakshasa protecting the camp. Aswathama invoked shiva and shiva gave him the sword and told him that with the help of the sword he can kill anybody till next day morning.
Krishna was aware of the dance of fate, so he moved pandavas and satyaki to shore of ganges not known to anybody. When aswathama went to the camp, the rakshasa disappeared on seeing the sword.
Aswatham killed everybody in the camp. So at the morning of 19th nobody was alive but for seven people in pandavas side and three people in the side of guaravas.
However there is no clear reasoning given to the best of my knowledge in any mythological story on why krishna allowed the happening of 18th night.
Further when abhimanyu started for the war, subadra asked krishna to protect his progency which he promised. But subadra was not aware at that point of time that utara, abhimanyu's wife was pregnant with parishit. Subadra was supposed to be the incaranation of "sabari". Finally when parishit was killed by braham siras which was only known to aswathama, krishna gave life to the child on the satya that krishna is "nitya brahmachari"