The ancient scriptures speak a lot on divine weapons used in the earlier yugas. There are mention of Brahmastra, Rudrastra, Narayanastra etc. But, why these weapons do not exist in the kali Yuga?
All divya astras are weapons of a single divine energy " Tejas" or ﬁery energy.” . This energy is channelled into the physical world by their wielders, with the correct mantras. Through incantations (mantras)a divya astra is generally united with a normal weapon (sastra) typically arrows – which obviously makes the Divine astra very very effective. Divine weapons are closely connected with emotions such as rage and anger. When unleashed they invoke all the raw power of a warrior’s fury, infused with the tejas of the gods, and thus are capable of laying waste hundreds of opponents in one concentrated act of violence
Most divine weapons are named after the deities who preside over them, and, in general, they manifest the natural phenomena associated with the particular powers they reﬂect. For example, the "Agneya astra" (“Fire-weapon”) belonging to Agni, the god of ﬁre and the Vaarun astra (“Water-weapon”) belonging to Varun . a, the water deity, creates water. Thus, a large array of divine weapons exists, which produce storms, lightning, ﬁre, wind, typhoons, mountains, rain etc.
one needs "Tapas" and self "Tejas "both to acquire and employ divine weapons successfully.
But due to change in time (Kaliyuga) Power(bala), intuition (buddhi) ,Tejasa ,tolerance, Foresight (pratipatti) has declined and this consequently resulted loss in the potential life expectancy of humans. So in kaliyuga no one can employ divine weapons, This is because of a decline in e potential amount of tejas any given individual can acquire. Thus the divine weapons are withdrawn from circulation ,and returned to their Divine owners.**
Ref.-DIVINE WEAPONS AND TEJAS IN THE TWO INDIAN EPICS (PDF paper) by JARROD L. WHITAKER ,University of Texas at Austin.