In Mahabharata, Krishna didn’t help Abhimanyu

In Mahabharata, Krishna didn’t help Abhimanyu knowing about his past life. Why did he save his unborn child when Ashwatthama used the brahmastra? Krishna saved Parikshit because he was born with the destiny of ruling Kuru kingdom after Yudhishtir. Aswathama tried to disrupt the divine plan of establishing the empire of dharma. When Chandra dev agreed to send Varchas to be born as Abhimanyu it was on condition that his child should succeed the throne. Let this Varchas, therefore, go thither, but let him not stay there long. Nara, whose companion is Narayana, will be born as Indra's son and indeed, will be known as Arjuna, the mighty son of Pandu. This boy of mine shall be his son and become a mighty car-warrior in his boyhood. And let him, ye best of immortals, stay on earth for sixteen years. And when he attaineth to his sixteenth year, the battle shall take place in which all who are born of your portions shall achieve the destruction of mighty warriors. But a certain encounter shall take place without both Nara and Narayana (taking any part in it). And, indeed, your portions, ye celestials, shall fight, having made that disposition of the forces which is known by the name of the Chakra-vyuha. And my son shall compel all foes to retreat before him. The boy of mighty arms having penetrated the impenetrable array, shall range within it fearlessly and send a fourth part of the hostile force, in course of half a day, unto the regions of the king of the dead. Then when numberless heroes and mighty car-warriors will return to the charge towards the close of the day, my boy of mighty arms, shall reappear before me. And he shall beget one heroic son in his line, who shall continue the almost extinct Bharata race.' Hearing these words of Soma, the dwellers in heaven replied, 'So be it.' LXVII Adiparva So Parikshit should survive to rule despite Aswathama’s vain attempt to kill him with his Brahmashira. Krishna revived Parikshit from death to make the promise given by gods become true. Abhimanyu has to return to Chandra on the 13 th day after fighting a heroic battle in the absence of Krishna and Arjuna. Krishna did not let Arjun know the fate of Abhimanyu who entered the chakra vyuha alone.He kept him preoccupied with Samshaptakas through out the day again for the same reason ie to let destiny act according to predestined plan. Krishna declared before Aswathama The holy one said, "The fall of this mighty weapon will not be fruitless. The foetus will die. But being dead, it will live again and have a long life! The heroic Parikshit, attaining to age and a knowledge of the Vedas and the practice of pious vows, shall obtain all weapons from the son of Sharadvata. Having obtained a knowledge of all high weapons, and observant of all kshatriya duties, that righteous-souled king shall rule the earth for sixty years. Sauptika parva section 16. Supreme power won’t interfere in the flow of happenings unless it is too essential for them to change it. Gods generally work in sink with one another and let the boons or curses effect without fail. Krishna did not let Indra’s Shakti weapon given with a word fail instead he made Karna use it against Ghatotkacha. He knew Arjun had more powerful astras but he also was aware of Arjun’s thought process which won’t make him oppose his father’s word that may bring illfame to Indra. My answer is based on story of Vyasa Bharat