After Mahabharata war, only Arjuna, Ashwatthama and Krishna were alive, who had divine knowledge of astras. In those
Ashwatthama was cursed to be harshly sick, so he forgot all his knowledge of astras after the war.
Arjuna, he felt that this knowledge would harm people in the future and that's why he never shared it with anyone after war. When he hugged Krishna during his last times, forgot all his Astra Vidya. As Krishna mentioned that there was no use of this Knowledge after the war as Kaliyuga was coming.
Krishna himself did not give the knowledge of astras to anyone after that. And when he left earth, it was the start of Kaliyuga.
It was this reason the knowledge of all astras was vanished, because in this times people would have been using them to kill their personal enemies and ex 🙂.
We the humans of Kaliyuga are not capable of handling that much knowledge and intensity of the divine Astras. Even if we have it, we will definitely destroy our own world.
Edit :— Guys I missed a point and I read it in the comments below. Yes, God Parshuram, God Hanuman, Rishi Ved Vyas, Rishi Kripacharya and Vibhishan, all are alive and they have the divine knowledge of Astras. They are among the 7 chiranjeevis who are waiting for Lord Vishnu's 10th avatar Kalki. All the 7 Chiranjeevis are having divine knowledge of Astras and many other things. But their knowledge is hidden and only be given to Lord Kalki and not to anyone else.